Learning science : Part 3 : World of chemistry molecules, materials, air, water

LEARNING SCIENCE Part 3 The world of chemistry: Of molecules and materials, Air around us, All about water.

Indumati Rao C. N. R. Rao JAWAHARLAL NEHRU CENTRE FOR ADVANCED SCIENTIFIC RESEARCH, BANGALORE

About the Authors Mrs. Indumati Rao has received degrees from the Universities of Mysore, Kanpur and Purdue (U.S.A.) and certificate in Education from Oxford University (U.K.). She has worked for over four decades in the field of education. She has contributed to developing new methods of teaching and teaching aids. Mrs. Rao has worked as a geography expert in many institutions. She has participated in several teacher orientation programs conducted by the Karnataka State. She has authored books in Geography. At present, she is working as the Honorary coordinator of the multimedia group at Jawaharlal Nehru Centre for Advanced Scientific Research. C. N. R. Rao is Linus Pauling Research Professor at the Jawaharlal Nehru Centre for Advanced Scientific Research and honorary professor at the Indian Institute of Science. He was at the Indian Institute of Technology, Kanpur, and has been a visiting professor at University of California, Santa Barbara. He is a member of many science academies including the Indian National Science Academy, the Royal Society, London, U.S. National Academy of Sciences, Russian Academy, French Academy, Pontifical Academy and Japan Academy. He was President of the International Union of Pure and Applied Chemistry, and is now President of the Third World Academy of Sciences. He has received several medals and prizes which include the Marlow Medal of the Faraday Society, Centenary Fellowship of the American Chemical Society, Einstein Gold Medal of UNESCO, the Centenary Medal of the Royal Society of Chemistry, London and the Hughes medal of the Royal Society. He has published more than 1300 papers in the areas of Chemical Spectroscopy, Molecular Spectra and Chemistry of Advanced materials. He has authored 38 books and has been active in science education. He was awarded the Karnataka Ratna in 2001 by the Karnataka Government. He is the first recipient of the India Science Prize, the highest scientific award recently instituted by the Government of India, and the Dan David International Prize for Science (2005). He was recently awarded the “Chevalier de la Legion d’Honneur”, the highest civilian award of France (2005). He was also named the `Chemical Pioneer of 2005’ by the American Institute of Chemists, USA.

LEARNING SCIENCE Part 3

The world of chemistry: Of molecules and materials, Air around us, All about water.

Indumati Rao C. N. R. Rao With the assistance of Jatinder Kaur Sanjay S. R. Rao

JAWAHARLAL NEHRU CENTRE FOR ADVANCED SCIENTIFIC RESEARCH, BANGALORE - 560 064.

Preface Science has become a part of our lives. Applications of science have provided us many benefits, and a better quality of life. The world today uses a language which has a lot of science in it. Without knowing, we use many words and phrases derived from science. We are also becoming conscious of our environment as well as our economy. Science has much to do with both these aspects. It is, therefore, important to learn the language of science. Children and adults alike have to know the rudiments of science and must be able to use the language of science where necessary. They must be able to apply the lessons learnt from science in daily life. It is for this purpose that we have produced a book entitled “Learning Science” in four parts. The book has the following four parts: Part1:

Universe, Solar System, Earth

Part2:

The world of physics and energy - Learning physical principles

Part3:

The world of chemistry: Of molecules and materials, Air around us, All about Water.

Part4:

Biology and life

It describes various aspects of science in simple language. It is hoped that this will be useful to school children as supplementary reading material and to all others who want to learn science and partake in the excitement of this experience. Bangalore 2005

Indumati Rao C. N. R. Rao

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CONTENTS Preface The world of chemistry: Of molecules and materials Objectives

1.0 Why chemistry? 1.1 So, what is chemistry? 1.2 Chemistry in ancient India

2.0 Elements and symbols

(iii)

1

2 3 6

8

2.1 Elements through the centuries

9

2.2 Elements and their symbols

10

3.0 Mixtures and compounds

12

3.1 What is a mixture?

12

3.2 Separation of substances in a mixture 3.3 What is a compound?

14 15

4.0 Atoms and molecules

17

4.1 Dalton’s observations of the atom

17

4.2 What is the atomic number of an element? 4.3 How do atoms exist in nature?

20 22

5.0 States of matter

25

5.1 Properties of the different states of matter

25

5.2 Change of state 5.3 Types of physical changes

27 29

6.0 Water: the cradle of life on earth

33

6.1 Water - the unique liquid

34

6.2 Hard water 6.3 Decomposition of water

36 38

7.0 Preparation and properties of some gases 7.1 Hydrogen: the most abundant element in the universe 7.2 Oxygen: the breath of life 7.3 Carbon dioxide: the source of food 7.4 Carbon monoxide

8.0 Acids, bases, salts and valence

39 39 42 46 50

52

8.1 What are acids?

52

8.2 8.3 8.4 8.5

55 56 56 57

Uses of acids What are bases? Examples of bases neutralising acids How can we identify acids and bases?

8.6 Salts 8.7 Valence

9.0 Carbon: the black rock that burns

59 60

62

9.1 Carbon - element with many forms

62

9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9

63 66 67 70 71 72 76 78

Diamond and graphite Combustion What is spontaneous combustion? Anatomy of a flame Carbon compounds Carbon and fuel gases Carbon: The backbone of polymers The world of plastics and fibres

10.0 Man and metals 10.1 Where are metals found? 10.2 Difference between minerals and ores 10.3 Metallurgy: the science of extraction of metals from ores 10.4 Physical properties of metals

11.0 Man-made materials 11.1 Cement - the versatile building material 11.2 Glass through the ages

82 83 83 84 85

89 89 91

11.3 Ceramics or the potter’s clay 11.4 Soap, the familiar cleansing agent 11.5 Chemical fertilizers

95 96 99

12.0 Air Around Us

101

Objectives

101

12.1 What is air? What is air made up of? Finding the missing ingredient The other missing ingredient Composition of air: then and now

12.2 Properties of air Air occupies space Air exerts pressure How does the air get heated?

12.3 Air and life on earth Nitrogen The restless air Pressure and wind Air: the moving force around us

102 102 103 106 108

110 110 111 113

115 115 117 118 120

12.4 Weather Daily weather bulletin Temperature and weather Back to water

12.5 Weather Forecasting How is weather forecast? Modern methods of gathering data Conditions of the sky: Clear or cloudy? Weather and climate

12.6 Air pollution Changing atmosphere Greenhouse effect Comforts on earth and danger from above

121 121 122 123

125 125 125 128 129

130 130 131 132

13.0 All About Water

134

Objectives

134

13.1 Water: the most abundant substance on earth How is water distributed on earth? Oceans and us

135 136 137

13.2 Nature’s gift: The water cycle Scientific understanding of the water cycle Role of evaporation in the water cycle Condensation: transition from gas to liquid Back to earth: precipitation

13.3 Properties of water Physical properties of water Chemical properties of water

13.4 Salinity of water Why is sea water salty?

13.5 Water pollution Water pollution - what are the reasons? Crude oil spills and pollution of the oceans It is time that we stop polluting our water sources

13.6 Purification of water From water source to taps Uses of water Running water and energy Conservation of water

141 141 142 144 146

148 148 150

153 153

156 156 158 160

161 162 163 164 166

Learning Science

The world of chemistry: Of molecules and materials

Objectives The world (The Universe) is full of chemical substances, present in various forms. They may be gases, liquids or solids, but they are all chemicals. In this module, we will learn about molecules and matter, a few types of compounds and materials with different properties.

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1.0 Why Chemistry?

Why should we learn chemistry? We must learn chemistry because chemistry deals with substances and the world is full of substances. Chemistry is everywhere. It is in

the stars

the colours we see

the medicines we take

the smell and taste we experience

the food we eat

In fact, the universe is a limitless chemical factory.

Colours and chemistry Natural colours are due to the chemicals present in them. Pigments or colours are obtained from specific minerals. Colours of the gems are also due to the chemicals present in them. Some minerals are ground into fine powder and used in the manufacture of cosmetics, paints and inks.

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Smell, taste and chemistry The smell and taste of fruits and flowers are due to mixtures of a number of organic compounds present in them. Wherever some compounds are found, the smell will be the same. For example, the smell of freshly picked raspberry is partly due to ionine, an organic compound. Ionine is also present in freshly cut grass; an essential oil obtained from violets also contains ionine. Smell of banana is due to isoamyl acetate. Most fruits contain natural sugars and traces of potassium.

Stars Stars are made up of mostly hydrogen and helium. They also contain some other elements in small quantities. The brightness of a star is due to the continuous nuclear reaction that goes on in its core. During this reaction, hydrogen is converted to helium and tremendous energy is released. Stars shine due to this heat. Our sun is only an average star. Yet, the temperature at its core is ~ 15,000,0000C.

The universe is a chemical factory It is filled with an amazing variety of chemicals. Chemical reactions in the vastness of the universe are responsible for the birth of stars, the creation of the planets and life on our planet.

1.1 So, what is chemistry? Chemistry deals with structure, properties and transformations of substances. Substances interact with each other all the time.

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Chemistry in action around us Why do chillies burn our tongue? And, lemons smell fresh? It is because of specific chemicals in them.

Colour

Smell

Taste

Sound

Why do chillies burn our tongue? The burning sensation we experience is due to the chemical capsaicin in chillies. This chemical aids digestion by triggering the production of saliva and helps in removing waste products from our system.

Why do lemons smell fresh? The fresh smell of lemon is due, to some extent, to the chemical limonene. This ‘oil’ is present in the peel. This chemical compound gives the distinctive smell to many citrus plants. The sour taste is due to citric acid. Lemon is more sour than oranges as it contains more citric acid.

Chemistry in daily life Chemistry is used by us practically all the time and everywhere. Chemistry is used by

doctors to save lives. hospital technicians.

the waterboard to purify water.

the police to catch criminals.

farmers to grow more food.

cooks to make tasty food.

weavers to add colours.

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Chemistry to catch criminals Chemistry is often used to solve crimes. Soil samples from a suspect’s shoes or from the tread of the car tyre used by the suspect is analysed for pH value. This is compared with the pH of the soil at the scene of crime. The DNA (from hair, fingernail, saliva etc.) of the suspect is determined by a special technique. This is called ‘ DNA finger printing’.

Purification of water Raw water collected in the reservoirs has to be purified before it is fit for drinking. Raw water has turbidity, certain dissolved salts that cause hardness, bad odour and taste, and harmful bacteria. These have to be removed. Water goes through the processes of sedimentation, filtration, aeration, lime-soda treatment and chlorination to make it safe for drinking.

Venom, snake’s defense Snake’s venom is a poisonous chemical. It is produced in special glands situated behind its mouth. Snake’s fangs are hollow. A snake injects venom into its victim through its fangs. The protein in the venom affects blood circulation by causing swelling and bleeding. Interestingly, snake venom is not poisonous when swallowed because the digestive system cannot break down the protein.

Spider’s thread The silk-like threads of a spider’s web is made of a protein. This fine thread woven by a spider is stronger than a steel thread of a similar thickness. A spider weaves threads of different kinds, each suited for the purpose - firm dry threads for the spokes of its web and sticky threads to catch its prey.

How does a chameleon change its colour? A chameleon changes its colour to merge with the background. The chemical melanin, helps a chameleon to change its colour

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easily. Melanin-producing cells become active when the chameleon is angry or afraid. It disperses melanin granules to produce colours ranging from yellow to brown and black. When the granules come together again, the chameleon regains its original colour. Some chameleons can change their skin colour to blue and red. Melanin is responsible for human beings getting tanned in the sun.

1.2 Chemistry in ancient India Chemistry has been used by people in India from the ancient times.

vegetable dyes for painting on rocks.

dyes in textiles.

gold for making ornaments.

People in India used

iron for making implements and weapons.

alloys for making weapons and statues.

Alloys An alloy is a metallic compound having two or more elements. An alloy can also be a solution. The components of an alloy generally are themselves metals. Carbon, a non-metal being the exception. Alloys are produced by melting the required combination of metals. Alloys were known and used extensively in the ancient world. Brass (copper and zinc) and bronze were two important alloys used then. Steel is an alloy containing chromium, manganese, molybdenum, nickel, vanadium and boron. Copper-nickel alloy, bronze and aluminium alloys are used in making coins. Fusible alloys having low melting points are used as solder and fuses in electrical circuits.

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Dyes in the ancient world Organic dyes like indigo, madder (source of red) and woad can easily attach themselves to cloth. Indigo dye is extracted from the leaves of the plant indigofera, and the red dye from the madder plant. Madder plant has yellow flowers and red root. The dye is extracted from the root. Blue dye is extracted from the leaves of the woad plant (an old world plant). Dyes extracted from plants are called vegetable dyes. People obviously knew how to extract dyes from plants.

Colourful ancient world Saffron and turmeric were used to add taste and colour to food. Vegetable dyes were used to make colourful textiles. Red lead was used to add colour to perfume bottles made of clay. Tribal chiefs painted their faces to show their status and to frighten the enemy.

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2.0 Elements and symbols Understanding elements We have said that we must learn chemistry because it deals with substances. What are substances?

A simple answer to this question is ‘All substances are made of elements’. Then, what are elements? The ancient Greeks and Indians tried to find an answer to this question. Wet Greek ideas of an element: Ancient Greeks worried about the nature of elements for Water Air centuries. Finally, they came to the conclusion that Fire, Earth, Air and Water were the Hot matter Cold four elements and everything was made up of these elements in different proportions. Earth Fire Dry

Indian concept of an “vayu”- the air element: Thousands of “akasha”- the sky kilometres away, in an entirely different continent, Indian philosophers also came to a similar conclusion. According to the Indian “kshiti”- the earth idea, matter was made up of “tejas” - fire the following five elements – ‘akasha’ or the sky, ‘vayu’ or the air, ‘tejas’ “ap” - water or fire, ‘ap’ or water and ‘kshiti’ or the earth.

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Modern idea of an element Let us understand the nature of an element. Take a piece of copper. Draw it into a wire. Cut the wire into pieces. Crush the pieces to powder. All these contain atoms of copper. Similarly, gold contains only gold atoms and iron, only iron atoms.

Lavoisier (1743 - 1794) Lavoisier is regarded as the father of modern chemistry. He was one of the earliest to recognise the importance of quantitative measurements. He explained the nature of combustion, established that air consists of oxygen and nitrogen. Lavoisier published a list of ‘elemental substances’ in 1789. He prepared his list after conducting careful chemical decomposition and recombination reactions. This list of 23 elements is considered by many as the first list of elements. But, he included lime, alumina and silica – stable chemical compounds – and light and heat in his list of elements.

2.1 Elements through the centuries Very few elements were known upto the first century AD. However, the following elements were known before the first century AD: Sun

aurum

Au

gold

Moon

Mars

Venus

Saturn

argentum

ferrum

cuprum

plumbum

Cu

Pb

Ag

silver

Fe

iron

copper

lead

Jupiter

Mercury

stannum hydrargyrum

Sn

tin

Hg

mercury

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Elements known upto the twentieth century Let us list the number of elements known through the ages. 1700s 12 elements 1800s 32 elements 1900s 80 elements th In the 20 century, 114 elements were known. Several of these elements do not occur in Nature. They are made in the laboratory. Scientists of the U.S.A. and Russia made most of them. We can, therefore, consider chemistry, as we understand today, to be quite recent. The most abundant elements on our planet: All the 92 naturally occurring elements are not evenly O (21%) distributed on earth. In the earth’s atmosphere, the most abundant oxygen silicon elements are nitrogen N (78%) (78%) and oxygen (21%). In the earth’s crust the abundant elements aluminium are oxygen, silicon, aluminium and iron. others iron

2.2 Elements and their symbols

calcium

How are the elements represented? Each element has a specific symbol. Letters of the alphabet are used as symbols. Symbols make it easy to represent elements. H - Hydrogen , O - Oxygen, B - Boron, C - Carbon, S - Sulfur, N - Nitrogen Chemical reactions can be written easily using symbols. C + O2 CO2 carbon

+

oxygen

carbon dioxide.

Early symbols Alchemists, considered by many to be the earliest chemists, used pictorial symbols to represent some of the elements they used.

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A circle represented gold, a crescent represented silver and a triangle represented water. Alchemists considered water as an element.

Alchemists Alchemists believed that man and everything in nature including metals were always trying to become perfect. According to them, gold was the perfect metal as it did not tarnish in air, it resisted the action of acids and it was not affected when heated with sulfur. Alchemists believed that all metals were striving towards becoming gold. Alchemists’ chief concern was to turn base metals to gold. They failed in this task but were able to gather knowledge, build new types of apparatus and paved the way for the development of chemistry.

Symbols in use now Using pictorial symbols for all the elements was not practical. So, a practical set of symbols acceptable to chemists all over the world had to be devised. An international chemical code was devised in the following manner. Wherever possible, the element is represented by its first letter. H - for hydrogen, O - for oxygen, N - for nitrogen, S - for sulfur, C - for carbon and so on. If more than one element had the same first letter, a second letter was added to represent the element. For example, Cl - for chlorine, Ca - for calcium, Co - for cobalt, Si - for silicon. There was yet another problem in assigning symbols to elements. Some elements had different names in different countries. Let us see how this problem was solved. Symbols for elements with Greek or Latin names: Berzelius, a Swedish chemist, solved the problem by giving symbols from their original Latin or Greek names. For example, Antimony - stibium - Sb, Copper - cuprum - Cu, Gold - aurum - Au.

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Some elements have symbols from mythological characters. For example, Helium – He (from Helios), Thorium – Th (from Thor). Some have symbols of planets. For example, Plutonium – Pu (from Pluto), Uranium – U (from Uranus).

3.0 Mixtures and compounds We use a variety of substances in our daily life such as water, salt, sugar, milk and foodstuff.

How are the elements present in water, salt, sugar and air? Water, salt, sugar and air have two or more elements. Water has hydrogen and oxygen in the ratio 2:1. Common salt has sodium and chlorine in the ratio Na Cl 1:1. Sugar has carbon, hydrogen and oxygen in the ratio 1:2:1. We breathe air in which a number of gases are present. We can, therefore, say that air is a mixture of gaseous elements. Some of the above substances are mixtures and some are compounds. Air is a mixture but common salt is a compound.

3.1 What is a mixture? Let us examine a few things we use in our daily life.

Wheat with other grains

Salt water Sugar in water

Milk

Sugarcane juice

These are all examples of mixtures.

Rice with small stones

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Let us prepare a few mixtures Take fine sand in one beaker and salt (or sugar) in another. Pour them into a third beaker and shake it well. You have a mixture of sand and salt. You can increase or decrease the quantities of salt or sand. It will still be a mixture of sand and salt.

Let us prepare a mixture in water Dissolve two tablespoons of sugar or salt in water. You get sugar or salt solution. Now you have prepared a mixture in solution form. Take some mud from your garden. Put it in a beaker containing water. Stir it well. You have a mixture where the solid particles of clay are dispersed in water. The solid particles do not dissolve in water (as in the case of sugar). This is also a mixture.

Types of mixtures Consider the two mixtures, sea water, and salt and sand. In sea water, salts are completely dissolved in water. Sea water is, therefore, a salt solution. In the mixture of salt and sand, we can still see the two separately. Sea water is a homogeneous mixture. Salt and sand is a heterogeneous mixture.

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3.2 Separation of substances in a mixture Substances in a mixture can be separated in a variety of ways. The choice of the method depends upon the nature of the mixture and the components in it. Some of the methods used to separate mixtures are:

Winnowing

Evaporation

Sieving

Distillation

Sedimentation

Sublimation

Decantation

Filtration

Making and separating a mixture You are given 2 tablespoons of sulfur powder, some iron filings, 2 tablespoons of sugar and 2 tablespoons of sand. First, make a mixture of these substances. Now, how will you separate the substances? Select the correct methods from those given below: Sedimentation, Filtration, Decantation, Evaporation, Loading, Sieving and Distillation. Repeat the same activity with copper sulfate, sulfur powder, iron filings and gravel. Copper sulfate Iron filings Sulfur Gravel

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Distillation Distillation was an important part of alchemy. The apparatus used to carry out distillation was called a still. The idea of still is believed to have been developed from the observation of a familiar object – the underside of the lid of a pot collecting the condensed liquid! Alchemists made the still from glass. The still was usually green because of the impurities present in the glass.

Separation of a mixture by loading Loading is used to speed up the process of fine suspended particles (especially in muddy water) settling down. A piece of alum is used for this purpose. Alum dissolves in water and the dissolved particles of alum ‘load’ the fine particles of clay. As a result, they become heavy and they settle down quickly. The clear water is then decanted. Similarly, dust particles in the air settle down as rain drops load the dust particles.

3.3 What is a compound? How is it different from a mixture? We will find the answer to this question through an experiment. Mix some iron filings with 2 tablespoons of sulfur powder. Bring a magnet near the mixture. The iron filings stick to the magnet. Using a magnet, remove all the iron filings. Sulfur powder is left in the plate. This was a mixture of iron and sulfur. Iron retained its property and sulfur remained sulfur. The properties of the substances in a mixture do not change. Now, gently heat the mixture of iron filings and sulfur powder in a test tube. After sometime, a coloured substance is left behind. Bring a magnet near it. The magnet does not attract the iron in the new substance.

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Cut the substance into smaller pieces and test them with the same magnet. The magnet has no effect. Iron loses its property in the new substance. This is a compound called iron sulfide containing iron and sulfur. The same elements, iron and sulfur, can form both mixtures and compounds. Can you think of other elements which forms both mixtures and compounds? Compounds contain two or more elements. The elements chemically combine together to form a compound with different properties.

Characteristics of a compound Potassium Copper sulfate A compound is made up of two chromate or more elements. The constituent elements in a compound do not retain their Sodium chloride individual properties. Sodium hydroxide The combining elements in a Iron sulfide compound are in a definite proportion. The elements in a compound cannot be easily separated.

Some examples of compounds H2O

Water

KCl

Potassium chloride

HCl

Hydrogen chloride

KOH

Potassium hydroxide

H2S

Hydrogen sulfide

CaO

Calcium oxide

HNO3

Nitric acid

CaCl2

Calcium chloride

H2SO4

Sulfuric acid

Ca(NO3)2

Calcium nitrate

NaCl

Sodium chloride

CaCO3

Calcium carbonate

NaNO3

Sodium nitrate

Ca(OH)2

Calcium hydroxide

Na2SO4

Sodium sulfate

CaSO4

Calcium sulfate

Na2CO3

Sodium carbonate

AlCl3

Aluminium chloride

NaOH

Sodium hydroxide

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4.0 Atoms and molecules Atoms: the building blocks of matter The study of the atom began in the 5th century BC itself. Democritus of Greece, described the atom as ‘that which cannot be cut up further’. Kanada of India defined the atom ‘as eternal and indestructible and which cannot exist in the free state’. However, it was more than a thousand years later, that a better understanding of the atom became possible.

4.1 Dalton’s observations of the atom Dalton observed that matter was made up of tiny indivisible particles or atoms, atoms of an element were identical and differed from the atoms of another element, atoms took part in chemical Na + Cl NaCl reactions atoms could neither be created nor destroyed.

Dalton (1766 - 1844) John Dalton, son of a poor weaver, began his career as a village school teacher at the age of 12 years! He became the principal of the school seven years later. In 1793, he moved to Manchester to teach physics, chemistry and mathematics in a college. Dalton proposed his atomic theory in 1803. He published a table of atomic masses. The errors (due to errors in his assumptions) in his first table were corrected in 1858. Dalton was the first to put the atomic theory on a quantitative basis.

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From his youth to his death, Dalton carefully recorded each day the temperature, pressure, amount of rainfall. He was a meticulous meteorologist! Dalton could not see red colour due to protonopia. This visual defect is known as daltonism.

Nature of an atom What do atoms look like? Till recently, people believed that atoms could not be seen. But now, under powerful microscopes, atoms can be seen and photographed!

Structure of an atom – modern concept

Silicon

According to modern ideas, an atom has a Protons central nucleus with Neutrons protons and neutrons in it. Protons are positively Helium atom charged. Negatively charged electrons revolve around the Number of protons = 2, Number of neutrons = 2, nucleus. Neutrons have no Number of electrons = 2. charge. Atoms are neutral because the number of protons in an atom is equal to the number of electrons in it.

Atom as a solar system The atom has been compared to the solar system. In an atom, the nucleus is the sun, the electrons are the planets. Just like planets in the solar system, the different electrons revolve around the nucleus in their own different orbits.

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How far are the electrons from the nucleus? To get an idea of this, let us imagine that we can enlarge the nucleus of an uranium atom so that its diameter is one metre. Then, the nearest electron would be 1.2 kilometres from the nucleus. The outermost electron would be 8 kilometres from the nucleus. The space between the electrons is empty as the space between the planets is empty in the solar system. Yet, uranium is an extremely hard solid. How does this happen?

Moving electrons Electrons move with enormous speed (~32,000 km/s). But their orbits are very small. Different electrons in an atom may move in different orbits. These orbits are called shells.

Shells

Let us look at the structures of the atoms of a few elements The number of electrons, protons and neutrons in the atoms of some elements. e

p

n

Hydrogen

1

1

0

Helium

2

2

2

Carbon

6

6

6

Sodium

11

11

12

Oxygen

8

8

8

Chlorine

17

17

18

Uranium

92

92

146

e - electron, p - proton and n - neutron.

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4.2 What is the atomic number of an element? The number of protons present in the Neutron nucleus of an atom is called the atomic number of an element. It is represented as Z. Electron The atomic number also indicates the number of Proton electrons present in an atom. Z = the number of protons = the number of electrons. Example: the atomic number of oxygen Sodium atom is 8 and that of sodium is 11. Oxygen atom

What is the mass number of an element?

The mass number of an element is the sum of the number of protons and neutrons present in the nucleus. It is represented as A. A = the number of protons + the number of neutrons. As a rule, the atomic number of an element is given below the symbol of the element (as subscript) and the mass number of an element is given above (as superscript). A where, E represents the element, A = the mass number Z and Z = the atomic number.

E

1

Example:

H1

12

C6

14

N7

23

Na11

Atomic and mass numbers of a few elements Element Hydrogen Helium Lithium Beryllium Boron Carbon

Symbol H He Li Be B C

Atomic number 1 2 3 4 5 6

Mass number 1 4 6 9 10 12

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Element Nitrogen Oxygen Fluorine Neon Sodium Magnesium

Symbol N O F Ne Na Mg

Atomic number 7 8 9 10 11 12

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Mass number 14 16 19 20 23 24

Atomic mass (weight): Can we weigh an atom? As an atom is extremely small, it is not possible to establish its weight (or mass) directly. The atomic mass or weight of an element is established indirectly. Dalton selected hydrogen as the standard and assigned its atomic weight as 1 (hydrogen being the lightest element). The atomic masses or weights of all other elements are greater than 1. Compared to hydrogen (atomic weight 1), the atomic weight of oxygen is 15.88. The atomic weight of oxygen was later fixed at 16. This was taken as the standard atomic weight as oxygen readily combines with other elements and oxygen compounds can be analysed easily. Also, as hydrogen is such a light element, errors can occur while calculating the atomic weights of heavier elements.

Atomic weights of elements taking oxygen as the reference Atomic weight of oxygen is 16.00. According to this, the atomic weight of hydrogen is 1.008. As this was also found to be unsatisfactory, the atomic mass (or weight) of carbon of atomic mass 12 (12C) is taken as the reference for defining atomic masses of elements. Atomic mass (weight) =

mass of one atom of the element 1/12th of the mass of one atom of 12C

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The atomic weights of a few elements are given below. Element

Symbol

Atomic mass

Hydrogen Helium Lithium Boron Carbon Nitrogen Oxygen Fluorine Sodium

H He Li B C N O F Na

1.00794 4.002602 6.941 10.811 12.011 14.00674 15.9994 18.99840 22.989768

4.3 How do atoms exist in nature? Atoms of many elements (especially + gases) cannot exist independently. Atoms exist in nature only if they Hydrogen Hydrogen Hydrogen combine with other atoms of the same atom atom molecule element or atoms of other elements. However, this is not true + + of noble gases (He, Ar, Ne etc.). Two Hydrogen atoms

Oxygen atom

Water molecule

When atoms combine, they form molecules. Molecules contain two or more atoms. A molecule of ammonia

Types of molecules Molecules can be formed by the same element or by different elements. In a molecule, there can be one atom – monoatomic – He, Ar, K, Li two atoms – diatomic – O2, H2, Cl2

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three atoms – triatomic – O3 (ozone) more than three atoms – polyatomic - S8 (sulfur), P4 (phosphorus). The number of atoms in a molecule of an element is called the atomicity of that element.

Molecule of a compound A molecule of a compound can contain atoms of different elements. A molecule of water is written as H2O. That is, in one molecule of water, there are two atoms hydrogen (H2) and Oxygen atom one atom of oxygen (O). One molecule of Hydrogen atom common salt is written as NaCl. That is, in one molecule of common salt, there Sodium Chlorine are one atom of sodium (Na) and one atom of chlorine atom atom (Cl).

Examples of some chemical compounds

Sodium chloride (NaCl) Copper chloride (CuCl2) Nitric acid (HNO3) Copper sulfate (CuSO4. 5H2O)

Potassium dichromate (K2Cr2O7)

Sodium hydroxide (NaOH)

Potassium permanganate (KMnO4)

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The molecular weight (mass) of a compound is obtained by adding the atomic weights (masses) of all the constituent atoms. For example, molecular weight of H2 = 2 x 1.008 = 2.016. Molecular weight of H2O = 2 x 1.008 + 15.999 = 18.

Molecular weights of a few compounds Compound

Chemical formula

Molecular mass (weights)

Water

H2O

18

Ammonia

NH3

17

Sodium chloride

NaCl

58.5

Carbon dioxide

CO2

44

Sulfur dioxide Hydrochloric acid

SO2 HCl

64 36.5

Nitric acid

HNO3

63

Sulfuric acid

H2SO4

98

Potassium hydroxide

KOH

56.1

Magnesium sulfate Potassium permanganate

MgSO4 KMnO4

120.4 158

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5.0 States of matter Look around you. You are surrounded by a variety of substances. Some of them are solids. Some are liquids. A few of them are gases. Matter can exist as a solid, a liquid or a gas. Most substances can exist in all the three states. Name a few gases that we commonly encounter.

5.1 Properties of the different states of matter Solids, liquids and gases differ in their properties. Let us see how they differ with respect to shape and volume. Shape: Solids have definite shapes. Their shapes remain the same. Liquids take the shapes of the containers. Gases do not have definite shapes. Volume: The volume of a solid does not change. The volume of a liquid remains the same. Gases do not have a fixed volume. They occupy all available space.

Why does a piece of stone not flow like water? Stone is a solid and a solid does not flow because it is rigid. In a solid, the atoms or molecules are closely packed. This results in strong forces of attraction holding the atoms or molecules together and minimum movement of molecules as there is little space between them.

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Properties of solids The closely packed arrangement of atoms or molecules makes the solids strong, hard and rigid (it can be compressed very little). Solids change to liquids when heated. Solids have definite melting points. Ice melts at 00C and iron melts at 15400C.

Why does a liquid change its shape? A liquid does not have a definite shape because of the way molecules are arranged in it. In a liquid, the atoms or molecules are loosely packed. This results in large spaces between molecules and weak forces of attraction between molecules.

Properties of liquids Liquids flow, take the shape of the container, can be compressed easily. When heated, liquids become gases. Liquids have definite boiling points. The boiling point of water is 1000C. Why does milk flow faster than honey?

Special features of gases Gases cannot be contained. Gases occupy all available space. can be compressed easily, have to be stored only in stoppered containers.

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Why do you smell a flower from a distance? Smell of flowers is due to the chemicals present in them. The molecules of these chemicals spread through the molecules of air. We can smell flowers or for that matter any other smell from a distance because of this molecular motion. This process is called diffusion. Diffusion can take place in liquids also.

What happens to a gas when it is heated? A gas does not change either to a liquid or to a solid on heating. It only gets hotter and expands. However, when cooled sufficiently, gases change to liquids.

Why is there humidity in the air? Water can exist as a solid, a liquid and a gas at the same time. It is this unique property of water that has made life on earth possible. There is always some gaseous water or water vapour over liquid water or even solid water. That is why there is always some water vapour in the air. Water vapour present in the air is called humidity.

Why do we smell camphor? Camphor is solid. We can smell camphor because there is some camphor vapour present above solid camphor. This is an example of a gas above a solid. Make a list of substances over which the vapour of the substance is present at room temperature.

5.2 Change of state Physical change Let us look at water again. Water, when heated to 1000C, changes to steam.

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The change of liquid (water) to gas (steam or water vapour) is called evaporation. Evaporation takes place from all moist surfaces wherever sufficient heat is available for this change to take place.

Back to water Cover a pan with boiling water with a lid. Remove the lid after a few minutes. What do you notice? There are water droplets on the underside of the lid. Water vapour or steam has changed back to liquid water. This change is the opposite to evaporation. It is called condensation.

Ice to water to ice Ice melts to form liquid water. Liquid water when cooled forms ice. Water when heated to 1000C becomes steam. When steam is cooled, it changes to water. These changes can be represented as follows: Water Ice Water Steam Whenever a change is represented by double arrows, it indicates an equilibrium.

Equilibrium Generally, when substances undergo chemical changes, products are formed. However, sometimes the products give back the original substances. Such a reaction is called a reversible reaction and the reactants and products are said to be in equilibrium. Example:

H2O + CO2

H2CO3

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5.3 Types of physical changes Physical change may be: a change of state (a solid to a liquid, a liquid to a solid or a gas, a gas to a solid), or an expansion of a substance when heated. (solids, liquids and gases expand when heated and regain their original size or volume on cooling).

Characteristics of a physical change In a physical change, no new substance is formed. we can restore the substance to its original state. the chemical composition of the substance remains the same.

How does water change from one state to another? For ice to melt to water or water to change to water vapour, we have to supply heat or thermal energy. On the other hand, for water to change to ice, we have to remove heat or cool it. Such changes occur when the temperature is increased (as in the change of water to steam). decreased (as in the change of water to ice).

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Let us look at the sequence of changes in the states of water

supply of energy

supply of energy

(heat absorbed)

Water

Ice

Steam

Cool (heat given out)

Cool

Ice

Water

Boiling water over a flame You must have seen water being heated on a flame. The flame may be due to burning wood, burning LPG or burning coal. Let us examine the change that takes place due to the burning of wood, LPG gas or coal. Wood, LPG and coal are carbon containing substances. Carbon burns in air to form carbon dioxide. A completely new substance is formed. The change that takes place in wood, LPG or coal cannot be reversed. Hence, this is a chemical change. When we see water boiling in a vessel over a flame, we are observing both a physical change and a chemical change. When we see a candle burning, we are observing both physical and chemical changes taking place simultaneously.

Features of chemical changes Chemical changes are the result of chemical reactions. The most important features of chemical changes are:

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the substances get transformed, the changes or transformations are permanent, the transformations may produce heat and/or light, a gas may be liberated or the substance may change colour, new compounds are formed as result of the chemical reactions.

Chemical reaction A chemical reaction is a process in which the starting substances are changed into different substances. That is, the products are different from the reactants or the starting substances. The individual properties of the starting materials disappear and the products have their own properties. However, the mass of the products remains equal to the mass of the reactants. A chemical reaction involves either the absorption of energy (endothermic reaction) or the release of energy (exothermic reaction).

Representation of chemical changes How can we represent the change that occurs when a candle burns in the language of chemistry? When carbon burns in air we get carbon dioxide. This is shown by the equation

C

+

O2

Carbon

+

Oxygen

CO2 Carbon dioxide

Similarly, when sulfur burns in air, we get sulfur dioxide.

S Sulfur

+

O2 Oxygen

SO2 Sulfur dioxide

The reactions are written using chemical formulae. Some other examples of chemical reactions and their representation by chemical formulae are:

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Magnesium burns in oxygen and gives magnesium oxide

Mg +

½ O2

MgO

H2 +

½ O2

H2O

Similarly, We can also write this equation as

2H2 +

O2

2H2O

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6.0 Water The cradle of life on earth

Water and the origin of living organisms Living organisms started in the oceans. Even now, the tropical rain forests bustle with life, but the tropical deserts are practically without life. From microscopic unicellular organisms to the gigantic multicellular organisms including the mammals and human beings can grow and reproduce only if water is available. The chemistry of water is closely tied to the chemistry of life. The unique properties of water are responsible for this. Water is practically everywhere. It is found in the flowing rivers, the frozen Antarctic, air, as water vapour. How does it do this?

It is possible because of the unique chemical bonds between hydrogen and oxygen.

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6.1 Water - the unique liquid Water is represented by the formula H2O. The beauty of water as a substance is that the water molecules (H2O) exist in different ways in steam, liquid water and ice. In ice, the water molecules form well-defined structures with some empty spaces or channels in them. That is why the density of ice is less than that of liquid water. In the liquid, there is little empty space. Remember that ice floats on water. Why are the properties of water, ice and steam different? The different properties of water as a solid (ice), liquid (water) and gas (steam) arise because the molecules of water get bound to one another differently in these three states. In ice, H 2O molecules bond with one another in a highly ordered fashion. water molecule

In steam, H 2 O molecules tend to exist somewhat as free molecules. In liquid water, H2O molecules are still bound to one another, but are not as ordered as in ice. Hydrogen atom

Oxygen atom

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Important physical properties of water Water is a powerful solvent. It has high surface tension. Water has adhesion property which helps a narrow capillary to pull the water upward against gravity. It dissolves both sugar and salt. Salt (NaCl) has ions of sodium and chlorine but sugar has molecules. Salt solution

Sugar solution

Chemical properties of water Water has a simple structure made up of two atoms of hydrogen and one atom of oxygen. Water acts as a kind of a magnet; the oxygen atoms attract electrons more than the hydrogen atoms do. Therefore, the oxygen atoms acquire a slight negative charge and the hydrogen atoms acquire a slight positive charge. The oxygen end and the hydrogen end of a water molecule resemble the two poles of a magnet.

Unique properties of water and their usefulness to life Polarity: Many kinds of chemical reactions take place in water medium. High specific heat: This helps in stablising the body temperature and the temperature of the surroundings. High heat of vapourization: Helps the evaporation process and cooling of body surfaces.

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Lower density of ice: Lakes do not freeze completely in winter. Adhesion: Water from the roots is pulled upwards and seeds germinate.

6.2 Hard water Water is the greatest solvent. It dissolves a variety of substances. Water in the rivers dissolve salts in the rocks over which they flow. Also, water seeping through the soil dissolves the soluble substances present in it. This makes water hard. Hardness of water is caused by the presence of these dissolved salts: Calcium bicarbonate Ca(HCO3)2, magnesium bicarbonate Mg(HCO3)2 Calcium chloride CaCl2, magnesium chloride MgCl2 Calcium sulfate CaSO4, magnesium sulfate MgSO4

Different types of hard water Hardness of water can be either temporary or permanent. Temporary hardness is caused by the presence of bicarbonates of calcium and magnesium. Permanent hardness is caused by the presence of chlorides and sulfates of calcium and magnesium.

Softening hard water Temporary hardness can be removed by boiling. When temporary hard water is boiled, calcium carbonate or magnesium carbonate is formed. This settles down as a precipitate.

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2HCO3 2- (aq)

37

CO32- (aq) + H2O (l) + CO2 (g)

Ca2+ (aq) + CO32- (aq)

CaCO3 (s)

How can we remove permanent hardness of water? Permanent hardness of water cannot be removed by boiling. Let us see how we can remove permanent hardness by conducting a simple experiment. Take two identical glass beakers. Name them beaker 1 and beaker 2. Fill 100 ml of 1 2 1 2 tap water in both the beakers. Add 20 ml of calcium chloride solution (or a little solid CaCl2) to both the beakers. Mix well. Add about 20 ml of sodium carbonate solution 1 2 1 2 (or a little Na2CO3) to beaker 2. Add about 10 ml of soap solution to both the beakers. Shake them well. What do you notice? There is no lather in beaker 1. Do you know why? Permanent hardness of water can be removed by the addition of sodium carbonate. However, this does not remove the hardness completely. To Hard Na2CO3 completely remove the water salts causing hardness, hard water is passed through the pores of zeolites. Zeolites are silicates or aluminosilicates. Hardness of water must be completely removed for use in hospitals and laboratories.

What are the disadvantages of using hard water? Soap does not form lather in hard water. It forms a precipitate or scum. As a result, dirt sticks to clothes. Hard water causes boiler scales.

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If temporary hard water is used in boilers (or water heaters), carbonates of calcium and magnesium get deposited as scales in the boiler tubes (and in the water heaters). This results in waste of heat energy as more heat is required to heat the water and the scales block the pipes and do not allow free flow of water in the pipes.

6.3 Decomposition of water How can we break water into its elements? Water is a stable compound. It cannot be broken into its elements (hydrogen and oxygen) by heating. It can be chemically decomposed by passing electricity through it. This process is called electrolysis.

Electrolysis of water Water can be decomposed into hydrogen and oxygen by electrolysis. Small pieces of aluminium foil are soldered to the ends of the two copper wires. The wires are then bent into a beaker. Water is poured into the beaker until the aluminium terminals are covered. A little sulfuric acid is added to the water as pure water is a poor conductor of electricity. Test tubes filled with water are inverted over the terminals (or electrodes). The test tubes are held in place by supports. The free ends of the copper wires are Anode Cathode connected to a source of current. After sometime, a gas collects at each electrode. The volume at one of the electrodes (at the cathode) is twice the volume of the gas collected at the other electrode. Test the gas for oxygen.

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7.0 Preparation and properties of some gases 7.1 Hydrogen: the most abundant element in the universe Did you know that hydrogen is present in the sun and the stars? The number of hydrogen atoms in the sun and the stars is far more than the number of atoms of all the other elements combined. However it is a different story on the earth. On the earth, hydrogen is ninth in the list of elements found in abundance. Let us find out why. Compared to the stars, the earth is a tiny astronomical body. Therefore, its comparative gravitational force is also very weak. When the earth was formed, it was not able to hold such a light element. As result, most hydrogen escaped into space.

Preparation of hydrogen Hydrogen is prepared by the displacement of hydrogen from dilute hydrochloric acid by metals such as zinc and magnesium. The reaction is given by Zn + 2HCl

ZnCl2 + H2

Dilute hydrochloric acid Mg + 2HCl MgCl2 + H2 and zinc The reaction takes place at room temperature. The liberated hydrogen is collected over water.

Hydrogen

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Physical properties of hydrogen Hydrogen is

a colourless and odourless gas

the lightest gas known

combustible inflammable

almost insoluble in water

Chemical properties of hydrogen Hydrogen burns in air to form water. It gives a pop with a lighted splint. It reacts explosively with oxygen. This reaction is used in hydrogen-oxygen fuel dil. H2SO4 cells to generate electricity. Platinum electrodes Hydrogen is liberated from acids by many metals.

Mg (s) + 2HCl (l) CuO H2

MgCl2

+ H2 (g)

Hydrogen flame Steam and unreacted hydrogen

Hydrogen is an excellent reducing agent. It removes oxygen from oxides.

CuO (s) + H2 (g)

Cu (s) + H2O (l)

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Compounds of hydrogen Hydrogen has the lowest atomic weight. It forms a number of compounds with many elements. Hydrogen is present in all compounds essential for the living world. It is present in:

starches (C6H10O5)x

sugars (C6H12O6)

x = a large number

proteins

fats

Hydrogen and hydrides Hydrogen combines with carbon to form hydrocarbons. (CH4, C2H6, petroleum, and natural gas). nitrogen to form ammonia. N2 + 3H2 2NH3 sulfur to form hydrogen sulfide. H2 + S H2 S chlorine to form hydrogen chloride H2 + Cl2 2HCl In fact, all acids have hydrogen.

Important uses of hydrogen Hydrogen is used in oxy-hydrogen torch used for welding, the preparation of ammonia and methyl alcohol CO + 2H2 the hydrogenation of oils to fats, the conversion of coal to lubricating oils.

CH3OH

Oxy-hydrogen torch The oxy-hydrogen torch consists of two tubes. These tubes conduct hydrogen and oxygen separately to the single jet where the gases are mixed and burnt. Hydrogen is first lighted and then oxygen is slowly fed to the flame. The

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flame becomes hotter and smaller. The temperature of this flame is around 2500-3000 0C. This torch is used in industry to cut and weld metals.

Hydrogenation Hydrogenation is a chemical process where liquid oils are converted to solid fats. In this process, hydrogen is added to oils in the presence of a catalyst. Inedible oils or less useful oils are converted to edible, hard fats by this process. For example, cotton oil is treated with hydrogen at about 5000C in the presence of a catalyst. The resultant hard fat is used for cooking. Fish oil is also converted to hard fat by a similar process and the resultant fat is used in the manufacture of soap.

Coal to lubricating oil Coal is converted to liquid fuels and lubricating oil by the addition of hydrogen or by the hydrogenation of coal. Powdered coal is mixed with tar. This mixture is treated with hydrogen under pressure. A chemical reaction takes place forming liquid fuels and lubricating oil.

7.2 Oxygen: the breath of life Oxygen is the most abundant element on our planet. It is in the atmosphere in a free state (Air contains ~21% oxygen). In the hydrosphere, oxygen is the most abundant element. (Water contains ~90% oxygen). In the earth’s crust oxygen is found in the form of oxides.

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Discovery of oxygen

Priestley

Priestley was an unsuccessful clergyman but a successful experimental chemist. He discovered oxygen towards the end of the 18th century. He conducted a simple experiment to prepare the gas which was later called oxygen by Lavoisier. Oxygen in the Greek language Lavoisier means acid producer.

Priestley’s experiment During Priestley’s time, the bunsen burner was not yet invented. He used the heat of the sun to provide the necessary heating for his experiment. He inverted a glass vessel filled with mercury and a small amount of mercuric calx or mercuric oxide in a trough containing mercury. He concentrated the heat from the sun on mercuric oxide by using a burning glass or lens. The heat decomposed the mercuric calx and a gas collected in the glass vessel. A candle burned brightly in this gas. However, it was Lavoisier of France who gave the name oxygen to this gas.

Preparation of oxygen Oxygen is prepared by the decomposition of hydrogen peroxide (H2O2) in the presence of a catalyst (manganese dioxide (MnO 2)). The reaction is as follows:

2H2O2

MnO2

2H2O +

Hydrogen peroxide solution

O2

The liberated oxygen is collected over water. The reaction occurs at room temperature.

Manganese dioxide

oxygen

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Collection of oxygen Oxygen is collected by the displacement of water. When the first bottle is filled with the liberated gas, it is replaced by another bottle containing water. The first bottle is discarded because it contains some amount of air. Hence it does not contain pure oxygen. When the second bottle is filled with oxygen, it is put on a gas plate while it is still under water. The bottle is removed from water and inverted while keeping it covered by the glass plate all the time.

Physical properties of oxygen Oxygen is a colourless gas an odourless gas a tasteless gas. It is slightly soluble in water.

Chemical properties of oxygen Oxygen combines with both metals and non-metals to form oxides.

C carbon

+ O2 oxygen

2Ca + O2

CO2 carbon dioxide

2CaO

calcium

oxygen

4Fe

+ 3O2

2Fe2O3

oxygen

ferric oxide

iron

calcium oxide

Oxygen supports combustion. It becomes very reactive at high temperatures. The reaction with hydrogen can be explosive.

2H2 (g) + O2 (g)

2H2O (g)

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Oxygen – Carbon dioxide cycle in nature During respiration in humans and animals, the inhaled air comes into contact with the blood in the lungs. Oxygenated blood is transported to the cells where it is used for the oxidation of food to carbon dioxide, water and other products. C6H12O6 + 6O2

6CO2 + 6H2O + heat energy

(glucose)

Carbon dioxide is transported back to the lungs by blood, where it is liberated and exhaled. The oxidation reaction in the cells liberates energy which keeps us warm and enables us to do work.

Uses of oxygen Oxygen is used for aiding respiration in seriously ill patients. by deep sea divers. by mountaineers. in fuel cells. in oxy-acetylene torch. Oxygen is needed for combustion. Combustion of coal, oil, wood and other fuels provides heat and electricity to operate automobiles, furnaces etc. C6H10O5 + 6O2 6CO2 + 5H2O + heat energy (cellulose)

CO2 + heat energy C + O2 Oxygen is used in steel industries in the blast furnaces.

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Why is pure oxygen given to seriously ill patients? Inhaled air enters the lungs. Oxygen present in the air passes through the lung membrane and enters the blood stream. If the lungs are damaged or its capacity is reduced by lung illnesses like pneumonia, asthma, oxygen is not transferred to the blood in sufficient quantities. In heart patients, not enough blood is pumped to the lungs for the gas exchange to take place. In such cases, pure oxygen is given to the patients.

Oxy-acetylene flame Oxy-acetylene flame is a very useful tool in modern industry as it can cut through steel slabs and weld them together again. In the oxy-acetylene flame, oxygen and acetylene from separate cylinders are fed into a blow torch. The gases are mixed thoroughly and burnt at the tip of the blow torch. The products of this combustion are carbon dioxide, water vapour and heat. The heat released reaches 30000C to 35000C. As this temperature is well above the melting points of most metals, the torch is used for welding. For cutting steel slabs, the flame is directed to the steel slabs. A supplementary supply of oxygen is added to the flame by another jet which burns the steel slabs sending out a shower of sparks. This torch can be used under water also.

7.3 Carbon dioxide: the source of food Carbon dioxide - the building block of the food chain

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Carbon dioxide is the source of food for the whole living world. Are you surprised? Plants use carbon dioxide to prepare food for themselves. Food prepared by the plants is the starting point of the food chain. Carbon dioxide was discovered by Van Helmont towards the end of the 16th century. Black was among the earliest to prepare the gas. Carbon dioxide is a product of combustion. It is produced by the combustion of food in the cells of human beings and animals. by fermentation. by the action of acid rain on marble. by decaying vegetation. Carbon dioxide is produced whenever a carbon containing substance burns in excess of air or oxygen.

Preparation of carbon dioxide

dil. HCl carbon dioxide

It is prepared from marble chips (calcium carbonate) by the action of dilute hydrochloric acid. The reaction takes place at room temperature.

CaCO3 + 2HCl

marble chips (calcium carbonate)

CaCl2 + H2O + CO2

Properties of carbon dioxide Carbon dioxide dissolves in water to form carbonic acid.

CO2 carbon dioxide

+

H2O

H2CO3

water

carbonic acid

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It reacts with sodium hydroxide to form sodium carbonate.

2NaOH

+

CO2

sodium hydroxide carbon dioxide

Na2CO3

+ H2 O

sodium carbonate

water

Carbon dioxide reacts with oxides of calcium and magnesium to form their carbonates.

CaO + calcium oxide

CO2 carbon dioxide

CaCO3 calcium carbonate

Write the reaction for carbon dioxide with magnesium oxide. Calcium carbonate and magnesium carbonate are insoluble in water.

Why does lime water turn milky when carbon dioxide is passed through it? Lime water is actually calcium hydroxide. When carbon dioxide is passed through lime water, carbon dioxide reacts with calcium hydroxide to form insoluble calcium carbonate (CaCO3).

Ca(OH)2

+ CO2

CaCO3

+

H2O

This turns the lime water milky. When more carbon dioxide is passed through this solution, the solution again becomes clear. This is due to the formation of soluble calcium bicarbonate.

Carbon dioxide – the breath of life for plants Can you imagine what would happen if the carbon dioxide we exhale, had accumulated in the air all the time. We would suffocate to death in the process of living! How does the air cleanse itself of carbon dioxide? Plants cleanse the air of excess carbon dioxide. Plants covert CO 2 to glucose, a simple sugar, by photosynthesis. Carbon dioxide is as essential to plants as oxygen is for animals and human beings.

Oxygen

CO2

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How is carbon dioxide produced in the living organisms? Carbon dioxide is produced in the living organisms by the combustion of food. How much of carbon dioxide does a person add to the air around us? A person exhales ~ 500 ml of air every time she/he breathes out. Of the ~ 500 ml of air, around 20 ml is carbon dioxide.

Uses of carbon dioxide Carbon dioxide is used in making fizzy drinks, in fire extinguishers (as carbon dioxide does not support combustion), as dry ice. Dry ice is used for large scale refrigeration.

What is dry ice? Solid carbon dioxide is called dry ice. When carbon dioxide gas is compressed quickly, there is a sudden drop in the temperature of the gas. This drop in temperature changes the gaseous carbon dioxide to solid carbon dioxide or dry ice. At ordinary pressure, dry ice changes from solid carbon dioxide to gaseous carbon dioxide without going through the liquid phase. The low temperature of dry ice (-780C) and its property of remaining dry during evaporation makes it a good refrigerant. Dry ice is used in cold storages to preserve meat and other perishables. Dry ice can be made in small quantities in the laboratory by the following procedure: Invert a steel cylinder containing carbon dioxide. Tie a small black bag over the outlet valve. Open the valve for a few seconds to allow the gas to rush into the bag. Now, turn the bag inside out. You will see dry ice deposited there.

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Too much free carbon dioxide in the atmosphere is bad as it results in global warming.

7.4 Carbon monoxide Carbon monoxide is a poisonous gas. It is produced when carbon containing substances burn in insufficient air or oxygen. Its formula is CO. Compare this with the formula of carbon dioxide. As carbon monoxide is a highly poisonous gas, it is not prepared in the laboratory.

Where is carbon monoxide found? Traces of carbon monoxide are found in coal stoves, furnaces and exhaust gas of automobiles.

air CO2+ C coal

2CO

air

Why should we not leave a car engine running in a closed garage? The exhaust gas from a car has a high percentage of carbon monoxide. If the engine of a motor car is left running in a closed garage, carbon monoxide gas gets collected in the garage. When air containing excess of carbon monoxide is inhaled, both oxygen and carbon monoxide pass through the lung membrane. Carbon monoxide combines with the haemoglobin of the blood forming carboxy haemoglobin. Carbon monoxide is ~ 300 times more soluble than oxygen in blood. As a result, very little oxygen is dissolved in blood. The carbon monoxide-rich blood cannot support life. It is for the same reason that people sleeping in a closed room with a burning coal stove often die.

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Preparation of carbon monoxide Lassone prepared carbon monoxide for the first time in 1776. Carbon monoxide is prepared by the following methods: By passing carbon dioxide over red hot charcoal (carbon)

CO2

+

C

By burning carbon in a limited supply of air

2C

+

O2

Δ

2CO

Δ

2CO

By passing carbon dioxide over red hot zinc or iron dust

CO2

+

Zn

By passing steam over red hot coal

C

+

H2O

Δ

Δ

ZnO + CO H2

+ CO

This is an important method of producing carbon monoxide and hydrogen industrially. When steam is passed over red hot coke, a mixture of carbon monoxide and hydrogen is produced. This gas mixture is called water gas, an important gas fuel. The two gases are separated by liquefaction.

Properties of carbon monoxide Physical properties Carbon monoxide is a colourless and odourless gas. not very soluble in water. extremely poisonous. Chemical properties Carbon monoxide does not support combustion. It reacts with chlorine in the presence of sunlight to form phosgene.

CO

+ Cl2

Sunlight

COCl2

Phosgene is a very poisonous gas. Carbon monoxide is used to produce many other important chemicals in industry.

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8.0 Acids, bases, salts and valence Sting of bees and wasps Why does the sting of a bee or a wasp hurt us? What have acids and bases got to do with the sting? Bee’s sting is acidic. The pain you feel is due to an acid (methanoic acid). Wasp’s sting also pains, but it is due to an alkali. What are acids and bases?

8.1 What are acids? The term acid is derived from the Latin word acidus meaning sour. Arrhenius (1859-1927) of Sweden introduced the idea of compounds splitting into their constituent ions. Based on this idea, he was able to explain that the concentration of the hydrogen ions in water (aqueous solution) determined the strength of an acid. Arrhenius was awarded the Nobel Prize in 1903 for his work on ionization. Acids are classified as naturally occurring acids and mineral acids. Acids are found in a variety of substances in Nature. Some of the naturally occurring acids are given below.

Lemon - Citric acid

Yoghurt - Lactic acid

Apples - Malic acid

Vinegar - Acetic acid

Tree bark and tea - Tannic acid

Tomatoes - Oxalic acid

Aerated drinks - Carbonic acid

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Can you name some more natural acids commonly used by us? Some of the naturally occurring acids used by us Source Acid Grapes Peanuts Sour milk Proteins Tamarind, Gooseberries

Citric acid Folic acid Lactic acid Amino acids Tartaric acid

What are mineral acids? Some acids are made from minerals and are, therefore, called mineral acids. They are made on a large scale in factories. The important mineral acids are hydrochloric acid (HCl), sulfuric acid (H2SO4) and nitric acid (HNO3). Mineral acids can be concentrated acids or dilute acids. Concentrated acids contain very little water. Strong acids like hydrochloric acid, nitric acid and sulfuric acid dissolve completely in water. As a result, they produce a large concentration of hydrogen ions.

Properties of acids Acids generally have a sour taste. Acids are corrosive. Concentrated mineral acids destroy organic matter such as paper, wood, human skin, cloth.

Be careful while using concentrated mineral acids! Do not spill acids on your skin or clothes.

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Properties of acids Oxides of some of the elements react with water to form acids.

C

+

Carbon

O2

CO2

Oxygen

Carbon dioxide

CO2 + H2O

H2CO3

Carbon dioxide Water

S

+

Sulfur

Carbonic acid

O2

SO2

Oxygen

Sulfur dioxide

SO2 + H2O Sulfur dioxide

SO3

Water

H2SO3 Sulfurous acid

+ H2 O

Sulfur trioxide

Water

H2SO4 Sulfuric acid

Acids react with some salts to give certain gases.

CaCO3 + 2HCl

CaCl2

+ H2O + CO2

CaCO3 + 2HNO3

Ca(NO3)2 + H2O + CO2

This property is used in fire extinguishers. What happens when you add nitric acid to eggshells? Eggshells contain mainly calcium carbonate. When dilute nitric acid is added to eggshells, a gas is liberated. The gas is carbon dioxide. How can you prove this? Acids react with metals to give hydrogen. Metal + Acid Salt +

Zn

+

2HCl

Zinc

Dil. hydrochloric acid

ZnCl2 + Zinc chloride

Hydrogen

H2 Hydrogen

Some metals do not react with dilute acids. For example, gold does not react with acids.

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8.2 Uses of acids Acids have a variety of uses. They are widely used in industries. Nitric acid is used to make fertilizers. manufacture explosives. purify gold and silver. to make aqua regia.

What is aqua regia? Acids generally do not react with the noble metals like gold, platinum and silver. Even concentrated nitric acid alone or concentrated hydrochloric acid alone does not affect gold or platinum. But a mixture of concentrated nitric acid and concentrated hydrochloric acid in the ratio of 1:3 dissolves gold or platinum. This is because concentrated nitric acid oxidizes concentrated hydrochloric acid.

HNO3 + 3HCl

2H2O + NO + 3Cl

Atomic chlorine attacks noble metals forming their respective chlorides (which are soluble in water). Alchemists named this mixture aqua regia (the royal water) because of its ability to dissolve gold and platinum.

Uses of sulfuric acid Sulfuric acid is used to make fertilizers. in the manufacture of paints, drugs, detergents and artificial silk. in car batteries.

as a dehydrating agent. in petroleum industry.

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Uses of hydrochloric acid Hydrochloric acid is used in the purification of common salt. as a bleaching agent. to make metal chlorides. to make glucose from starch. as a cleansing agent and to make aqua regia.

8.3 What are bases? Bases are oxides or hydroxides of metals. Bases that dissolve in water are called alkalis.

Na2O + H2O K2O + H2O CaO + H2O

2NaOH (Sodium hydroxide) 2KOH (Potassium hydroxide) Ca(OH)2 (Calcium hydroxide)

Bases are used to neutralise acids. For example, milk of magnesia is used to neutralise acids in the stomach.

Properties of bases Bases and alkalis have a bitter taste, are soapy to touch, neutralise acids.

Ca(OH)2 + 2HCl Base

CaCl2 + 2H2O

Acid

They react with acids to form salt and water.

NaOH Base

+

HCl Acid

NaCl + H2O Salt

Water

8.4 Examples of bases neutralising acids Examples in daily life: Bases are used in the treatment of acidity in human beings. acidic soil insect stings.

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Other uses Weak ammonia solutions are used in household cleaners. Sodium hydroxide is used in the manufacture of dyes, soap, paper and artificial silks.

Uses of calcium hydroxide Calcium hydroxide or slaked lime is used in the preparation of plaster, bleaching powder, mortar.

bleaching powder mortar

plaster

Calcium oxide or quick lime is used in the treatment of soil.

8.5 How can we identify acids and bases? Acids and bases can be identified by their characteristic reactions with the indicators. Indicator

Acid

Base

Litmus

Red

Blue

Methyl orange

Orange

Yellow

Phenolphthalein

Colourless

Pink

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Litmus Litmus is a purple dye extracted from lichens and sea weeds. Lichens grow abundantly along the Mediterranean coast. Litmus readily dissolves in water to form a purple solution. This solution is used as an indicator to find out whether a solution is acidic or alkaline. Acid turns the litmus solution red and the base turns the litmus solution blue. Litmus paper is made by dipping absorbent paper in the litmus solution, drying it and cutting it into strips. Generally, litmus paper is used as an indicator as it is easy to use.

Estimating the strengths of acids and bases The indicators do not give an estimate of the strength of an acid or a base. To know the strength of an acid or a base an universal indicator can be used. A combination of indicators is used to make the universal indicator. The strength of an acid or an alkali is given by its pH value. The term pH is derived from a German word meaning strength of an acid.

pH scale Acidic

0

1

Strong

Alkaline

Neutral

2

3

4

5

6

7

8

9

10

11

12

Weak Weak

13

14 Strong

If a solution has pH < 7 it is acidic. If pH = 7 it is neutral and if pH > 7 it is alkaline.

Nature’s indicators - colours of hydrangea flowers Soils can be acidic or alkaline. Colours of flowers are due to the chemical compounds present in them. These compounds are often sensitive to the acids and alkalis present in the soil.

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For example, hydrangea flowers are generally white in neutral soil, pink in alkaline soil and blue in acidic soil.

8.6 Salts Salts are essential for all living beings. The most commonly used salt is the common salt or sodium chloride (NaCl). Other examples of familiar salts are: calcium sulfate (CaSO4), calcium chloride (CaCl2), zinc sulfate (ZnSO4) and potassium chloride (KCl).

How are salts prepared? Salts can be prepared by the reaction between an acid and a base. This reaction produces salt and water.

HCl

+

acid

NaOH

NaCl

base

salt

+

H2O water

The reaction between an acid and a metal produces a salt and hydrogen gas.

Zn metal

+

H2SO4 acid

ZnSO4 salt

+

H2 gas

Metals form a variety of useful salts. For example, most metals form their respective oxides, carbonates, chlorides, sulfates and nitrates. Acids form their respective salts. For example, Sulfuric acid (H2SO4) forms sulfates. (ZnSO4, CaSO4) Hydrochloric acid (HCl) forms chlorides. (CaCl2, KCl) Nitric acid (HNO3) forms nitrates. (KNO3)

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8.7 Valence Valence is defined as the combining capacity of an atom in forming compounds. To understand valence, we shall examine three sets of compounds:

1

2

3

HCl HBr H2O H2S

NaCl NaBr Na2O Na2S

FeCl2 FeO FeCl3 Fe2O3

Let us look at the compounds containing hydrogen in column 1. HCl

One atom of hydrogen combines with one atom of chlorine in HCl.

HBr

One atom of hydrogen combines with one atom of bromine in HBr.

H2O

Two atoms of hydrogen combine with one atom of oxygen in H2O,

H2S

Two atoms of hydrogen combine with one atom of sulfur in H2S.

If we take the valence of hydrogen as one, then, chlorine (Cl) and bromine (Br) will have valence of one, and oxygen will have valence of two. We take the valence of hydrogen (H) or oxygen (O) as the reference. What is the valence of sulfur? Two. We now look at column 2 with compounds of sodium (Na). NaCl One atom of sodium combines with one atom of chlorine in NaCl. NaBr One atom of sodium combines with one atom of bromine in NaBr. Since the valence of chlorine or bromine is one, the valence of Na (sodium) is one. Na2O That is why two atoms of sodium combine with one atom of oxygen in sodium oxide (Na2O). Na2S

Remember, sulfur has a valence of two.

Since Na has valence of one, all other elements of the same kind (that is, the alkali elements K, Rb, Cs) also have valence of one.

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Let us look at the iron compounds in column 3 FeCl2 One atom of iron (Fe) combines with two atoms of chlorine in FeCl2. Therefore, Fe (iron) has a valence of two (since valence of chlorine is one). FeO

One atom of iron combines with one atom of oxygen in FeO Therefore, here the valence of Fe (iron) is two just as in FeCl2 (Again Remember! Oxygen has a valence of two).

FeCl3 One atom of iron combines with three atoms of chlorine in FeCl3 Therefore, here Fe (iron) has a valence of three. Fe2O3 Two atoms of iron combine with three atoms of oxygen in Fe2O3. Thus, iron can have a valence of two or three. Valence of elements – conclusions All elements do not have the same valence. Some elements have more than one valence. Valence of hydrogen (1) or oxygen (2) is taken as the reference.

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9.0 Carbon The black rock that burns Carbon, the element with symbol C, gets its name from the Latin word “carbo”. Carbo in Latin, means a black rock that can burn! The word highlights a physical property (its colour) and a chemical property (its combustibility). Probably, carbo also referred to coal.

9.1 Carbon - element with many forms Carbon exists in nature in many forms.

They are: charcoal, diamond, deposits on spark plugs, coke, graphite and soot.

Carbon - the element essential for life Carbon compounds abound in nature and in living organisms. Carbon dioxide is in the atmosphere. Thousands of chemical compounds in our body have carbon as one of the elements.

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Carbon in the world around us Carbon in combination with other elements is found in

Common foods

Cellulose Rayon

Limestone, chalk

Fuels (wood, natural gas, gasoline etc.)

Proteins and fatty acids

Wood pulp

Yet, the total percentage of carbon in the atmosphere, hydrosphere and the earth’s crust put together is only 0.18.

Carbon in free state Carbon occurs in free state as

Diamond

Graphite

Coal

Charcoal

Lamp black

They all have different physical properties but have the same chemical properties. Even diamond and graphite burn in air to form carbon dioxide.

9.2 Diamond and graphite It is hard to convince many that the glittering diamond and the lead of the pencil are chemically the same. How are they different? Diamond Graphite glitters greyish black disperses light opaque hard to touch slippery touch

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The differences in the physical properties of diamond and graphite are because of the manner in which the carbon atoms are arranged. Their chemical properties, however, are the same. They both burn in air to form carbon dioxide. They are allotropes of carbon.

Allotropes of an element Some elements exist in various forms. These forms have different physical properties but have the same chemical properties. They are called allotropes of the element. Carbon, phosphorous and sulfur have allotropes.

Diamond - the hardest natural substance known Diamond is one of the purest forms of carbon. is represented by the same symbol as carbon (C). occurs in all shapes and sizes. is found in igneous rocks. Diamonds are formed under intense pressure and heat. It is found in the igneous rock and gravel, but is unevenly distributed. South Africa and Russia are the largest producers of diamond. The diamond mines at Kimberly in South Africa are famous. In fact, the diamond bearing rock is called Kimberlite. The weight of diamond is measured in carats (1 carat = 0.2 g). The largest diamond ever found weighed 3105 carats or 621g. Diamond is mined in India at Panna, Madhya Pradesh. The most famous diamond of India is the Kohinoor. It is the jewel of the royal crown of Britain.

Arrangement of carbon atoms in diamond In diamond, each carbon atom is joined to four carbon atoms. This gives diamond a three dimensional structure and hardness. As there are no free electrons, diamond is a bad conductor of electricity. It is a good conductor of heat.

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Uses of diamond When we think of diamonds we think of sparkling jewellery. Diamond has other important uses. Diamond tipped tools are used for cutting and drilling rocks. It is used in spacecraft to make radiation proof windows.

Arrangement of carbon atoms in graphite In graphite, each carbon atom is strongly bound to three other carbon atoms in the same plane. Carbon atoms are arranged in layers. As the layers are not held together, they can slide over one another. This makes graphite soft. It is a good conductor of electricity. How is pencil lead made?

Uses of graphite Graphite is used as a lubricant (combined with petroleum jelly) - graphite grease. as electrode in batteries and electric furnaces. as a crucible to melt metals (because of its high melting point - 35000C). for making pencil lead and in electric motors (carbon brushes).

Pencil lead Pencil lead is made of graphite. Graphite is mixed with wet clay in required quantities to give the required hardness. The pasty substance is then rolled and hardened by heat. Pencil lead or graphite has been used for writing since the sixteenth century. The word graphite is derived from the Greek word graphein meaning to write.

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9.3 Combustion Before we look at carbon and combustion, let us understand What is combustion? What are the conditions for combustion to occur? What are the products of combustion? Understanding the nature of combustion Discovery of fire changed human life irreversibly. Civilization began when man learned to build a fire. Primitive man did not know the cause of fire. Many superstitions about fire arose out of ignorance of the cause of fire.

What is fire? The cause of fire was a mystery for many centuries. Lavoisier of France finally solved the mystery of fire towards the end of the 18th century. He conducted a number of experiments and concluded that fire was the result of a chemical reaction between a burning substance and oxygen. This marked the end of alchemy and the beginning of the modern period.

What is combustion? Combustion is the chemical reaction between a combustible substance and oxygen. During combustion both heat and light are produced. Substances that burn are called combustible substances. All substances having hydrogen and carbon burn to give carbon dioxide and water. Metal and combustion Metals also burn in oxygen producing heat and light. For example, burning of magnesium ribbon.

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How does combustion start? Ordinarily, a combustible substance does not burn spontaneously. A combustible substance must reach the ignition or kindling temperature to start burning. External heat has to be supplied to start combustion. The temperature at which a combustible substance starts to burn is called its kindling or ignition temperature. Coal gas or LPG does not start to burn at ordinary temperatures. Kerosene also has a high ignition temperature.

9.4 What is spontaneous combustion? When coal is stored in warehouses, coal dust above the coal is slowly oxidized by the oxygen in the air in the warehouse. Heat is liberated during this reaction (exothermic reaction). If this heat raises the temperature of the air to more than the ignition temperature of coal, the coal dust bursts into flames, on its own. Spontaneous combustion is the combustion that occurs without the help of an external source of heat.

Grain warehouse fire Grain dust collects in the warehouse. The dust reacts with oxygen of the air. This reaction generates heat. The heat raises the temperature to ignition temperature and spontaneous combustion takes place. Sometimes, grain warehouse fires are caused by sparks.

Fire in an airplane factory Magnesium and aluminium are used extensively in making aircraft. In metal tool workshops in aircraft factories, magnesium and aluminium dusts get accumulated. The magnesium and aluminium dusts react with the oxygen in the air of the workshop and spontaneous combustion occurs.

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Oily rags and spontaneous combustion When oily rags are left in a closet or badly ventilated room the oil slowly oxidizes. The heat of the reaction cannot escape. When the kindling or ignition temperature is reached, the rags burst into flame.

Fire in a haystack The dust from the hay collects in the barn. The dust reacts with oxygen. The reaction generates heat which raises the temperature to ignition temperature. Spontaneous combustion occurs. When a haystack catches fire outdoors, the heat is liberated well inside the haystack. The heat is generated by the action of bacteria that decompose the hay. The heat released by this starts a fire.

Why do dust explosions occur? Tiny dust particles have large surface areas. As a result, reaction with oxygen takes place at a fast rate. A fair amount of heat is generated by the reaction. If the temperature reaches the ignition temperature of the dust particles, the dust particles burst into flame.

The great destructive fires Some of the great fires in history are Burning of Rome in 64 AD. Rome burned for six days, while emperor Nero played the harp. Great fire of London in the 17th century. This fire burned for 14 days! Chicago fire in the 19th century destroyed the city in a single night. How to extinguish fire was a problem. Carbon dioxide provided the answer.

Fire extinguishers Fires can be caused by ordinary combustible materials.

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gasoline oils and greases. short circuiting of electrical equipment. All fires are not alike. Each requires a different type of fire extinguisher.

Types of fire extinguishers The three types of fire extinguishers are Dry powder fire extinguisher for electrical fires. Foam type for gasoline fires. Acid soda fire extinguisher for fire from combustible material.

Dry powder extinguisher This extinguisher contains a mixture of sand and baking soda (NaHCO3). When this mixture is thrown over the fire, baking soda decomposes and releases carbon dioxide. Carbon dioxide extinguishes the fire.

Foamite extinguisher When a solution of sodium bicarbonate and alum (aluminium sulfate) are mixed, foam is formed. The foam is due to the carbon dioxide trapped in the form of bubbles. This is called foamite. The bubbles look like whipped cream or softy icecream. Its great advantage is that it sticks to both horizontal and vertical surfaces. The alum and soda solution are kept in separate containers. They mix only when the extinguisher is inverted.

Acid soda extinguisher In this fire extinguisher, carbon dioxide is produced by the action of sulfuric acid on a saturated solution of sodium bicarbonate. It has an iron cylinder with a flexible tube. An iron cylinder is filled almost full with sodium bicarbonate solution. Sulfuric acid, in a small glass bottle, is kept in the upper part of the cylinder. The bottle is well supported. When the cylinder is inverted, chemical reaction takes place and the released carbon dioxide gushes out of the flexible tube.

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Do you want to make a fire extinguisher? Take a bottle with a rubber stopper. Make a hole in the stopper so that a thin (8 mm) glass tube with a right angle bend passes through it. Make about 400 ml of 5% sodium carbonate solution. Pour the solution into the glass bottle. Fill a test tube with dilute hydrochloric acid and lower it into the bottle with a string attached to the test tube. Close the bottle with the stopper. Fix the tubing. Attach a rubber tube with a glass jet at the other end to the tubing. Use copper wiring to hold the rubber tubing and jet lightly. Your fire extinguisher is ready to use.

9.5 Anatomy of a flame Zone of non-combustion: Zone of complete Zone of partial combustion Combustion does not take place combustion here as there is no oxygen Zone of supply here. Blue Zone non-combustion Zone of partial combustion: Here, paraffin wax decomposes into free hydrogen and carbon. This is the luminous part of the flame. The flame is yellow due to the unburnt carbon particles. Blue Zone: is at the bottom of the flame. Combustion is incomplete here. The blue color of the flame is due to the formation of carbon monoxide. Zone of complete combustion: This is the outermost and the hottest part of the flame. The flame is non-luminous. It is invisible. Hydrogen and carbon combine to form carbon dioxide and water.

Combustion in the human body Combustion in the human body occurs in the cell. The food we eat is broken down into simpler substances (like glucose) in the digestive system. Glucose is the gasoline. Combustion can take place only if there is oxygen. When we inhale air, the oxygen in the air passes through the lung membrane and enters the blood vessels. Oxygen is carried by the red blood cells (haemoglobin) to the cells.

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Oxygen is released in the cells where it combines with carbon and hydrogen to form carbon dioxide and water. What is the difference between combustion in human beings and combustion outside? In ordinary combustion, carbon monoxide is always formed. Carbon monoxide is not formed during combustion in animal and human cells. How has this difference saved us?

9.6 Carbon compounds Carbon compounds abound in nature. Carbon forms oxides with oxygen,

2C + O2

2CO, C + O2

carbides with metals,

CaO + 3C

CaC2 + CO

hydrocarbons with hydrogen, CH4, C2H4, C6H6

CO2 H

H

C

C

Hydrocarbons - the energy compounds Methane (CH4), the simplest hydrocarbon, has one carbon atom and four hydrogen atoms. Methane is formed by the slow decomposition of organic matter in the absence of air. Methane is formed near swamps and waste dumps. in the intestines of cattle and bodies of termites. Methane is highly combustible. It burns in air to form carbon dioxide and water.

CH4 + 2O2

CO2 + 2H2O

Why do we see flames near the waste dumps?

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Fire near waste dumps Waste dumps have vegetable matter. The vegetable matter in the waste dumps is decomposed in wet conditions by the action of bacteria present deep inside the dumps. As a result methane is formed. This reaction produces heat. Methane gas catches fire. For this reason, methane is also called marsh gas.

Some familiar hydrocarbons A few other compounds of carbon and hydrogen are:

Ethane, C2H6

Ethylene, C2H4

Acetylene, C2H2

Benzene, C6H6

9.7 Carbon and fuel gases Carbon forms gases that can be used as fuel. Some important fuel gases are Coal gas: It is a mixture of many gases. It was used to light street lamps. Water gas: It is a mixture of carbon monoxide and hydrogen. H2 + CO C + H2O Natural gas: It is a mixture of 85% methane, 10% ethane and 5% other hydrocarbons. Natural gas is the cleanest and most efficient fuel gas.

Coal gas Coal gas is the oldest of the fuel gases. It was first produced towards the end of the 18th century. Oil lamps in Pall Mall (the famous street in London) was replaced by coal gas. In the U.S.A., Baltimore was the first city to use coal gas for street lighting. Gradually its true value as a fuel was understood. Coal gas is a mixture of hydrogen (~50%), methane (~35%). It is now used as a very important fuel.

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Carbon fuels - Coal, petroleum and natural gas Heat from burning these fuels made the wheels of modern industry go round. Coal made industrial revolution possible. Man’s skill in extracting petroleum and refining it made modern industry flourish. Natural gas is now replacing both coal and petroleum as the fuel of choice.

Coal: The black diamond You may be surprised that coal is compared to diamond. In many ways coal is more useful than diamond. There are two major coal forming periods: The carboniferous period (~500 million years ago). Coal fields of North America and Europe belong to this period. The Gondwana period. Coalfields of India belong to this period.

Dangers of coal mining Methane gas is found in pockets in the coal mines. The miners use a lamp for illumination and cut the coal seams using metal tools. Sometimes, the heat generated by this causes explosions in the mines. An explosion is always followed by the formation of carbon monoxide. Carbon monoxide is extremely poisonous when inhaled even in small quantities. This gas often prevents rescue work. Sometimes the mines are flooded, sometimes the roofs collapse trapping the miners. Miners continuously inhale the fine coal dust. This results in lung diseases.

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Coal - the versatile substance Bituminous coal is heated in the absence of air to remove moisture and other impurities. This process is called destructive distillation. The by-products of destructive distillation are coke, coal gas, tar, ammonia, benzene, toluene, xylene, naphthalene.

Destructive distillation When coal is heated in a closed vessel, it cannot burn because oxygen is lacking. Coal decomposes into a variety of substances like coal gas (gas), tar (liquid) and coke (solid). Industrial destructive distillation of coal is carried out in coke ovens. The crushed coal is heated to a very high temperature for about 18 hours. Coal decomposes into various products when coal undergoes destructive distillation.

Carbon and explosives The explosive TNT was widely used in World War 1. TNT or Trinitrotoluene is an unstable molecule. Its starting substance is coal tar. When subjected to a mechanical shock, atoms in this unstable molecule rearrange themselves to form carbon dioxide, steam and nitrogen gases. This causes a sudden expansion in its volume which in turn produces the destructive explosion.

Synthetic dyes W. H. Perkin (1838-1907) was the first to make an artificial dye. He was trying to prepare quinine from coal tar but he was disappointed to obtain only a dark sticky substance. When he was cleaning this sticky substance with alcohol, it dissolved in alcohol and formed a purple solution. He crystallized a synthetic dye for the first time. The textile industry was revolutionized by this accidental discovery. This dye was called ‘Mauve’. Alizarin, the chemical compound that gives the natural dye madder its red colour, was successfully synthesized from anthracene in 1868.

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The first “Mac” Thomas Hancock (1786-1865) and Charles Macintosh (1766-1845) a chemist dissolved natural rubber in naphtha obtained by the fractional distillation of coal tar. They, then dipped ordinary cloth in this rubber solution to produce waterproof cloth for making coats. This waterproof coat came to be known as “Mackintosh”.

Synthesis of new materials Chemists have synthesised many organic compounds mostly from water, air, coal and petroleum. These compounds have high molecular weight. Chemists have made synthetic rubber, leather, plastics and textiles. However, they have not yet succeeded in synthesizing the most complex compounds like cellulose, wool and silk and some specific proteins.

Petroleum - the most important fuel Petroleum oil has been known for centuries. Petroleum oil seeped through the earth in some places forming small pools of oil or sticky puddles of tar. In some places oil vapour and natural gas leaked through the cracks in the earth’s surface and caught fire.

Refining petroleum or crude oil Petroleum or crude oil is a thick, dark coloured liquid. It is a mixture of various hydrocarbons. Crude oil or petroleum cannot be used as it is. The various hydrocarbons have to be separated. Different hydrocarbons present in crude petroleum have different boiling points. The separation of the hydrocarbons is done by making use of this property. This process is called fractional distillation. Fractional distillation Fractional distillation is carried out in a tubular furnace with a fractionating column. Crude oil is heated in a furnace to ~8000C and the resulting vapours are passed into the fractionating column. The vapours condense in the

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ascending order of their boiling points. There are usually six outlets in the tower to collect 1. gasoline 2. kerosene 3. fuel oil 4. lubricating oil 5. vaseline, paraffin wax 6. asphalt and tar The products of fractional distillation are:

Aviation fuel Paraffin wax, asphalt and tar Fuel oil Kerosene

Gasoline

Vaseline

Lubricants

9.8 Carbon: The backbone of polymers What are polymers? Polymers contain repeating chemical units. The simple repeating chemical unit in a polymer is called a monomer. Mono means one, poly means many. For example, H2C CH2, ethylene, is a monomer of the polymer, polythene.

Hydrogen Carbon

Polyethylene or polythene

nCH2

CH2

(

CH2

CH2 )n

Different kinds of polymers Polymers can contain monomers of the same chemical unit, or monomers of different chemical units. Polymers can be naturally occurring or synthetic (or man-made).

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Hydrogen Carbon Chlorine

PVC ( Polyvinyl chloride)

nCH2

CHCl

(

CH2

CH

)n

Cl Carbon forms both biological polymers (natural polymers) and synthetic (man-made) polymers. Some carbon containing natural polymers are: Cellulose in plants, wood resins, rubber and starch. Protein DNA and proteins are natural polymers. Chemists have synthesised a wide variety of polymers.

Synthetics Of all the man-made materials, the synthetic materials are perhaps used most. What are synthetic materials?

Synthetic materials are generally organic compounds made from coal, petroleum and such starting materials. Many of them are polymers.

The versatile synthetics Synthetics have provided materials for many purposes. Shelter

Communications

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Many of the synthetic materials are not just substitutes for scarce natural materials. They are also competitors of materials like wool, silk, cotton, leather and rubber. Clothing

Transportation

Synthetic fibres from proteins Synthetic fibres made from proteins are: Aralac from milk protein casein, Vicara from corn protein zein, and Saralon from peanut protein. These fibres look like wool and can also be dyed just like wool or silk is dyed. These fibres are blended with wool to add strength.

9.9 The world of plastics and fibres Two of the most important synthetic materials are plastics and fibres.

Plastics

Fibres

This is the mental picture we get when we think of plastics. But glass becomes plastic when heated. Clay is plastic when wet. Plastic materials are actually substances that are soft (some only when heated) and can be moulded easily.

How are synthetic plastics made? John Hyatt was the first to make plastics. He made celluloid in 1868. Leo Baekeland made bakelite. Synthetic plastics are made from petroleum products. There are two types of plastics: thermoplastics and thermosetting plastics.

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Thermoplastics: Thermoplastics can be heated and cooled over and over again. Polythene, PVC and acrylics are thermoplastics. Thermosetting plastics: Thermosetting plastics can be heated and moulded only once. Bakelite is a thermosetting plastic. Chemists have also made the transparent plastic, lucite (plexiglas).

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linear

branched

crosslinking

John Hyatt John Hyatt was trying to make imitation ivory billiard balls from nitrocellulose and camphor. Instead of ‘ivory’, he got a new substance. He called it “celluloid”. Celluloid has serious disadvantages; it is brittle, inflammable and discolours with age.

Bakelite In 1909, Leo Baekeland, a Belgian chemist, patented the process for manufacturing a plastic material called bakelite. When phenol and formaldehyde are heated together in alkaline solution, they combine to form an amber coloured resin-like solid called resinoid. Resinoid is a plastic which can be melted. On further heating at higher temperatures, it changes to a material which cannot be melted. This material is called bakelite. Bakelite is used to make handles for utensils, spark plug covers, electrical insulators etc.

Lucite or transparent plastic This is also made from easily available raw materials like petroleum, natural gas, salt, coal and water. Lucite or plexiglas is a highly transparent substance. Its unique property is that light rays pass through a lucite rod even when it is bent and emerges at the other end. This property is used in surgery and dentistry. Light rays are directed to the affected part of a patient without any risk of causing burns to the patient. Lucite can be machined, sawed or drilled with ordinary tools.

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Lucite or plexiglas is used to make unbreakable lens, watch crystals, jewellery and safety glass for airplanes and automobiles.

Synthetic fibres Synthetic fibres may be made from cellulose or plastics. Hilaire Chardonnet made the first synthetic fibre from cellulose in 1889. Wallace Carothers made the first plastic-based synthetic fibre nylon in 1935. Ladies stockings were the first articles to be made of nylon. Second World War gave a boost to the production of nylon. Why? Nylon is used to make parachutes.

Synthetic fibres - How many kinds? Some of the popularly used synthetic fibres are

Rayon Nylon

Terylene Dacron

Acrylics

Nylon: the versatile fibre Nylon material is

Wrinkle free

Strong

Light and durable

Moth resistant

Moisture resistant

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Rayon Rayon is made from cellulose. Count Chardonnet made the synthetic fibre from mulberry leaves. Chardonnet was studying the diseases of the silkworms. He was inspired by the silkworm spinning silk to find a way to make artificial silk. He made artificial silk from the cellulose he obtained from mulberry leaves by a complex process.

Plastics - blessing or curse? Plastics have both advantages and disadvantages. Advantages of plastics: Plastics are inexpensive. are easy to make on a large scale. are light and do not corrode or rust.

do not conduct heat or electricity. are easy to mould into any desired shape. can be made colourful, and in some cases, can be recycled.

Problems with plastics When plastics have so many advantages, how can they be a curse? Plastics are not biodegradable (i.e., they do not decompose easily). pollute the environment. are inflammable. give off harmful fumes when they burn. (PVC gives off hydrogen chloride fumes and polyurethane gives off hydrogen cyanide and carbon monoxide fumes.) are harmful to animals (some animals eat soft plastic things which block their intestines).

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10.0 Man and metals Look around you. You will find metallic objects at home and elsewhere. At home you see many different metals: Aluminium and stainless steel vessels in the kitchen. Copper in the electrical wire. Iron in the window bars or grills.

Tungsten in electric bulbs. Silver and gold in jewellery. Chromium in cars.

Use of metals and alloys in Egypt and India Egyptians used gold, copper, iron and tin to make daggers, vases and mirrors. Both the Indians and Egyptians used brass and bronze. Indians made beautiful statues and other goods using these metals. How are brass and bronze different from other metals? These are alloys. It needed chemists to devise ways to make metals an essential and important part of our lives. Today, the amount of steel used in a country is taken as an indicator of how advanced it is.

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10.1 Where are metals found? Metals are found in the earth’s crust. Of the elements found in nature, about 70 are metals. Metals occur both in the free state (e.g. gold). in the combined state or as compounds (titanium as oxide, iron as oxide). Naturally occurring compounds of metals are called minerals. Minerals are mixed with useless rocky material, clay, sand etc. along with metals. These impurities together are called gangue.

Most abundant metals in the earth’s crust The earth’s crust does not contain equal amounts of metals. The most abundant metals are: Ore Metal Percentage Bauxite, Corundum (oxides) Haematite, Magnetite (oxides) Magnesite (oxide) Sodium chloride Potassium chloride

Aluminium

7-8%

Iron

5-6%

Magnesium Sodium Potassium

3% 2.5% 1.5%

Copper (0.01%), lead (0.04%) and zinc (0.002%) are called non-ferrous metals. By tonnage, the output of steel in industry is ~90%. The remaining 10% of the other metals are equally crucial.

10.2 Difference between minerals and ores Ores are minerals from which metals can be extracted. Ores may be Oxides - Iron, aluminium, copper. Sulfides - Copper, zinc, mercury. Sulfates - Calcium, barium, lead. Chlorides - Sodium, potassium, magnesium.

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We can extract metals from all the minerals. All ores are minerals but all minerals are not ores. Think about it!

How pure are ores? Minerals have a high percentage of impurities. The amount of gangue in all the mineral ores is not the same. As a result, the metallic content differs from one ore sample to another. High grade iron ore has 60% iron. Copper ore has 5% copper. Silver ore has only 1% silver. An ore may contain only one metal or several metals. Do you know that the iron ore (blue dust) in Kudremukh, Karnataka is almost pure iron oxide!

10.3 Metallurgy: the science of extraction of metals from ores Getting metals out of the ores was practised in many cultures. It is only in the last few centuries that scientists have found efficient ways of extracting metals. The main steps in extracting metals are: Concentrating the ore, Conversion of metal oxides, Reduction of the oxide to the metal and Refining the impure metal.

Extraction of metals Obtaining metals from their ores is called extraction. The method of extraction depends upon the chemical property of the metal. All metals cannot be extracted by the same method. The common methods employed are: 1. Electrolysis (when the metals are reactive) Potassium, Sodium, Calcium, Magnesium, Aluminium are obtained by this process. 2. Smelting: Iron is obtained by this method. 3. Roasting in air and smelting and blowing hot air - copper is extracted by this method.

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Iron Iron is the work horse of man. It is obtained by the reduction of iron oxide. The reduction is carried out Mixture of iron ore, coke and limestone in a blast furnace. The reduction takes place in stages. 3Fe2O3 (s) + CO (g) 2Fe3O4(s) + CO2 (g) Fe3O4 (s) + CO (g) 3FeO (s) + CO2 (g) FeO (s) + CO (g) Fe (l) + CO2 (g)

hot gases

hot air Molten slag Molten iron

hot air

10.4 Physical properties of metals Metals are generally solids. (Mercury is a metal but it is a liquid). have lustre. are generally hard. Lustre Hardness

Malleablility

Ductility

Good conductivity

are malleable. are ductile. are good conductors of heat and electricity. have high melting points, some have high density.

Chemical properties of metals Metals react with oxygen to form metal oxides. Metal + Oxygen Metal oxide Some metals react with oxygen at room temperature. Examples: Sodium, Potassium Magnesium and oxygen form magnesium oxide.

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Reaction of oxygen with zinc and lead

2Zn 2Pb

+ +

O2 O2

2ZnO 2PbO

Copper forms copper oxide.

2Cu

+

O2

2CuO

However, gold and platinum do not react with oxygen. They are noble metals.

Metals and water Sodium, potassium and calcium react vigorously with water at room temperature.

Na sodium

+

H2O

NaOH

water

sodium hydroxide

+

H2 hydrogen

Magnesium reacts violently with steam. Gold, silver and copper do not react with water.

Reaction of metals with acids Most metals react with dilute acids. Salt and hydrogen are produced. Metals + Acid (dilute) Salt + Hydrogen Keeping in mind the way sodium and potassium react with oxygen and air, predict the reaction of sodium and potassium with dilute acids. Which metals do not react with dilute or concentrated acids?

Metals displace other metals Look at the following reactions:

Zn + Mg +

CuSO4 CuSO4

ZnSO4 MgSO4

+ +

Cu Cu

In both the reactions, copper has been replaced (by Zn and Mg). Now consider the following reaction.

Cu + Cu +

ZnSO4 MgSO4

No reaction No reaction

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Copper is less reactive than either zinc or magnesium. A more reactive metal will always displace a less reactive metal from its salt solution but the opposite reaction is not possible. Reactivity of metals can be arranged in an order (Reactivity series).

Metal oxides and water Oxides of metals dissolve in water to form metal hydroxides. Metal oxide + water Metal hydroxide

4Na Na2O

+ O2 + H2O

2Na2O 2NaOH

Write the equation for calcium oxide and water. This hydroxide is called lime water. Lime water is used to test the presence of carbon dioxide. Magnesium oxide reacts with water to form magnesium hydroxide.

MgO

+

H2O

Mg(OH)2 (milk of magnesia)

Milk of magnesia is taken to neutralise acidity in the stomach.

Why do metals corrode? Have you wondered why iron articles turn brownish, copper vessels turn green and aluminium articles become dull? This is because iron, copper and aluminium form oxides when exposed to air. Both oxygen and moisture in the air are responsible for the corrosion of metals. Gold and platinum do not corrode.

How to prevent corrosion? Corrosion can be prevented by the following methods:

Painting the metal

Anodizing

Oiling and greasing

Do you know ships are protected by “sacrificial protection”?

Galvanization

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Sacrificial protection Corrosion is a major problem of ships as iron corrodes easily. However, magnesium corrodes more easily than iron. Therefore, a bar of magnesium is attached to the side of a ship. The magnesium bar, instead of the exposed part of the ship, corrodes. This is called sacrificial protection as magnesium is sacrificed to protect the ship. When it is completely corroded, the magnesium bar is replaced. Zinc can also be used for the same purpose.

Mineral wealth of India India is rich in some essential minerals. Some of them are: Ilmenite, Pyrolusite, Bauxite, Haematite and Monazite sand.

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11.0 Man-made materials Some of the man-made materials and chemicals used in our daily life are

Glass

Soaps

Cement

Ceramics

Pesticides, fungicides, herbicides.

Detergents Fibres

Fertilizers

11.1 Cement - the versatile building material History of cement The early Egyptians used a building material very similar to cement. Romans used a building material obtained by mixing lime and volcanic ash. They used it to pave roads and build other structures throughout Europe. Cement from limestone and clay was manufactured only in the 19th century.

Portland cement Aspdin of England made cement using limestone and clay for the first time in 1824. He mixed limestone and clay, burned the mixture in a furnace and ground

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it to a fine powder. When this powder was mixed with water and allowed to stand, it became a hard yellowish mass. This cement was stronger than all the other cements produced till then. He called this the Portland cement as it looked like the limestone found near Portland. Aspdin patented the process. What are the raw materials required for making cement? The basic ingredients for making cement are calcium carbonate, silica, alumina and iron oxide. All these are found in limestone (calcium carbonate) and clay (silica, alumina and iron oxide).

Manufacture of cement Steps in the manufacture of cement are: Lime stone and clay are mixed in correct proportions. This mixture is crushed to powder in a rotating cylinder. The powder is heated in a special rotating kiln to a very high temperature. The mixture forms lumps. The lumps are called clinkers. Clinkers are cooled and mixed with gypsum. The mixture is ground to a fine powder in special machines. The quality of cement depends upon its fineness.

Different kinds of cement Oil well cement is designed to withstand high pressure and temperature of the oil wells. Water proof cement is used in water storage tanks and roofing of houses. Gypsum cement is used in plasters and plasterboards. Slag cement made with slag of blast furnaces, can resist erosion by chemicals. Cements have also been made from natural resins and synthetic materials (plastics).

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What is concrete? Concrete is a paste containing cement, sand and crushed rock (usually granite) or gravel and water. Water reacts with cement to form a sticky substance which binds the sand and gravel together. A synthetic rock is formed. This hardens with time. As concrete remains soft only for a short time, it is moulded into the required shape. Concrete can withstand pressure but it is not flexible.

What is reinforced concrete? Have you seen a house being built? Steel rods or wiremeshes are inserted in the concrete moulds or concrete is poured over steel rods or meshes. This is called reinforced concrete. This is done as ordinary concrete cannot withstand pressure and cannot stretch (or is rigid). Only reinforced concrete is used in bridges and buildings.

11.2 Glass through the ages Glass was used for a variety of purposes even during the ancient times. The Egyptian glass beads from ~ 2500 BC are the earliest glass objects known. Glass was used by Alexandrians of Ptolmey’s times. Ancient Romans. Egyptians from as far back as 2500 BC. Syrians of the first century AD. Modern glass was first made in Alexandria.

What does glass contain? Ordinary glass contains silica. Silica is sand. Different materials are added to silica to get different types of glasses.

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How is glass made? Glass is made from silicates. Silicates have high melting points. Sodium carbonate and calcium carbonate are added to bring down the melting point. The mixture is heated in a furnace to ~14000C. The transparent and jelly-like substance is cooled slowly and then blown into desired shapes.

All glasses are not the same Glass is an important material. In the ancient world, glass was used mainly to make artistic objects or attractive containers to hold medicines and perfumes. Now, glass is used for many purposes.

A wide range of glasses with specific properties are now made by adding specific substances.

How does glass get colour? Specific chemical salts are added to glass before it cools down completely to get different colours. Chemical salts and coloured glass: Cobalt oxide gives purple or blue coloured glass. Copper oxide gives red coloured glass. Chromium salts give green or yellow coloured glass. Other coloured glasses are made in this manner.

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Coloured glass Colours of coloured glass depend generally upon the metallic oxide. Some metallic oxide may give different colours or different metallic oxides can give the same colour depending upon the glass mixture. Colouring agent Ferrous oxide Ferric oxide Lead Selenites and selenates Copper Excess copper

Colour Pale blue or olive green Yellow (may turn pale blue or olive green) Pale yellow Pink or pinkish yellow Peacock blue Green

Different types of glass Plate glass: Plate glass is much thicker than ordinary glass. It is made by floating a layer of molten glass over molten tin. Plate glass has a very smooth surface. It is used in shop windows and doors. Optical glass: Optical glass is a special type of glass. It is made by a process so that the glass is free from defects and strains. It is used for making lenses for microscopes, cameras, telescopes and correction spectacles. Bullet proof glass: This is also called laminated glass. It is made by binding several layers of safety glass with transparent adhesive. The strength of the bullet proof glass depends upon the number of layers of safety glass. Cracks produced by any impact on the surface of the bullet proof glass ends at the adhesive. This glass is used in airplanes, windshields of automobiles and bullet proof screen. Photochromatic glass: This type of glass is sensitive to sunlight. It temporarily darkens when exposed to bright sunlight. When the intensity of sunlight decreases it comes back to its original shade. This is due to the

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presence of silver iodide in the photochromatic glass. This glass is used in making sun glasses, sunshields.

Fibre glass, optic fibres and glass wool A special property of glass is that it can be drawn into fine fibres when it is in plastic state. Glass fibres combine the properties of fibres (they can be woven into cloth) with the properties of glass (they are bad conductors of heat and electricity).

Glass with plastics Glass fibres are combined with plastics to get strong materials. These are called reinforced plastics. These materials are used for roofing. building boats. sports equipment. making suitcases and boxes. automobile bodies.

What is glass wool? Glass wool is a bundle of loose glass fibres. Glass wool can trap air. Therefore, it is a good insulator. Because of this property, glass wool is used in refrigerators. ovens. cookers. and hot water bottles.

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Optical fibres - the wonder fibre of the modern world You may have seen workmen laying optical cables in your neighbourhood. These cables are made of special glass fibres. The glass fibres are coated with a special material. Optical cables act as a tunnel for the passage of light signals over long distances. Optical fibres - the information highway The ability of optical fibres to transmit light signals has revolutionised the method of transmitting information and medical diagnostics. Now, doctors can actually peep into the faulty part of your body and repair it without a major surgery.

11.3 Ceramics or the potter’s clay Ceramics derives its name from the Greek word keramos (potter’s clay). Ceramics or potter’s clay has been used since prehistoric times to make a variety of objects. Clay is formed from the weathering of igneous rocks containing the mineral, feldspar. Clay occurs abundantly in nature and is found in all soils. Only its percentage differs. Clay contains fine silica, alumina and iron oxide. It can be moulded into any shape.

Traditional ceramics Ceramics made in the traditional method are hard but brittle. are porous. can absorb water. can withstand high temperatures. can be glazed.

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Porcelain or white ceramic Porcelain is a special kind of ceramic. Porcelain is made from pure white clay and other materials. It is translucent, waterproof and is used for making chinaware.

Properties of ceramics Ceramics are poor conductors of heat. They can withstand high temperatures. poor conductors of electricity. very hard (example - silicon carbide).

11.4 Soap, the familiar cleansing agent We use soap in a variety of ways during the day.

We also use different types of soaps for different purposes.

Perfumed soap

Liquid soap

Washing soap

Shaving soap

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What is soap? Soap is the sodium salt of a fatty acid (stearic acid). Fatty acids have a long hydrocarbon chain with an acidic group (COOH) at one end. Soap is made by the reaction between stearic acid and sodium hydroxide. Sodium hydroxide + Fat Soap + Glycerine. The chemical name for soap is sodium stearate (C17H35 COONa).

Soap - the two headed molecule Soap molecules have two ends: the sodium end and Na+ COOHthe hydrocarbon end Na+ Na+ Na+ The sodium end has affinity for water or is Na+ Na+ hydrophilic but has no affinity for oil or grease. + Na Na+ The hydrocarbon end repels water or is Oil hydrophobic but has affinity for grease or oil. Na+ Na+ Na+ This unique property of soap helps it to remove Na+ + + Na Na dirt.

The cleansing action of soap You may wonder what the unique property of the soap molecule has got to do with the cleansing action of soap. When soap is applied to a wet soiled surface, the sodium end of the soap molecule is attracted to water and + COOHthe hydrocarbon end Na is attracted to the grease. As a result, the dirt particles are pushed away from the soiled surface. They stick together and form scum. The scum is then rinsed off.

Soap and hard water When clothes are washed in rivers or ponds, the dirt is not washed off and the soap does not form lather. The river water or the water from the pond often has

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dissolved calcium and magnesium salts in it (or the water is hard). The sodium ions react with these dissolved salts. Sodium stearate + Hard water (soap)

Calcium stearate + Magnesium stearate (scum)

As a result, the grease and dirt are not pushed away from the surface. If the water in your area is hard, add some sodium carbonate to water before washing your clothes.

Manufacture of soap The demand for soap increased after the second world war. The reasons are: There was an improvement in the quality of life. People became more aware of cleanliness and hygiene. With the increase in population in the developing world, the demand for soap is increasing. As a result, manufacture of soap on a large scale has become necessary.

Detergent - the soapless soap The increased demand for soap tested man’s ingenuity. The answer to the increased demand was soap made from petroleum hydrocarbons - the detergent. A detergent is also a two-headed molecule: one hydrophilic and water attracting the other hydrophobic. But the detergent has no fatty acid.

Detergent or soap - which is better? Detergents are more advantageous than soap in many ways but the cleansing action of both are similar. Detergents do not give a precipitate in an acidic solution. are effective even when used with hard water. They do not form scum. increase the penetrating power of water. Therefore, they are good wetting agents. Detergents are ideal for washing laboratory glass ware.

water repelling

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11.5 Chemical fertilizers Plants need nitrogen, phosphorus and potassium for their growth. The soil is normally able to provide these nutrients to plants. When the same crops are grown over and over again, the soil loses these essential nutrients. Chemical fertilizers help to sustain or in some cases revive the fertility of the soil.

All chemical fertilizers are not the same The different types of chemical fertilizers are Nitrogen containing fertilizers: The basic substance in nitrogen fertilizers is ammonia. Nitrogen containing fertilizers are ammonia, urea, ammonium sulphate, ammonium nitrate and ammonium phosphate. Nitrogen fertilizers are essential for increase in food production. Phosphorus containing fertilizers: Phosphates are the basic substances in these fertilizers. Phosphorus is also essential for plants. Phosphorus compounds are produced by powdering rock phosphate and adding sulfuric acid to it. The resulting products - phosphoric acid and calcium salts (from the rock) are directly applied to the soil to increase its phosphorus content. Potassium fertilizers: examples: Potassium chloride, Potassium nitrate etc. A complete fertilizer contains nitrogen, phosphorus and potassium in a fixed ratio. This is called NPK fertilizer.

Protecting plants from diseases: Role of pesticides and insecticides The health of plants gets affected by viruses. bacteria. fungi. insects. Pesticides and insecticides are used to protect plants.

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Pesticides - boon or curse? Pesticides have certainly helped in increasing food production. However, their continued use in large quantities leads to long range problems as they are toxic chemicals. The plants absorb the pesticides and as a result they enter the food chain. Pests also become immune to them. Their gradual accumulation in the human body causes health problems. DDT and BHC have been banned in many countries for these reasons.

DDT - the chemical that protects and kills DDT is very effective in controlling pests. But it is not biodegradable. As a result, it gets into the food chain. DDT causes long term damage to animals and human beings.

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12.0 Air Around Us

Objectives  It is important to know about the air we breathe.  We learn about air, the atmosphere and weather in this module.  Air pollution is also part of this module.

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12.1 What is air? The ancient Greek philosophers were sure that air contained only one substance, and called it an element. Indian philosophers also had the same idea and called it vayu. But everyone agreed that the air around us was like an invisible ocean enveloping both the seas and the land.

What is this invisible ocean made up of? Air-a mixture of gases. The Greek idea of air survived for many centuries. However, in 1770, Scheele, a Swedish scientist proved that air  was not a single element. inactive air active  was a mixture of active air air and inactive air and  the mixture had 80% inactive inactive air active air air and 20% active air. 80% 20% But this was not the end of the story.

Air around us Scheele proved that air was a mixture of active air and inactive air. He rusted iron in a closed bottle. He showed that air consisted of 80% of inactive air and 20% active air. Lavoisier (in the 18th century) called them nitrogen and oxygen. Towards the end of the 19th century, Ramsey showed that inactive air also contained 1% of other gases. The analysis of air which began around 500 BC was completed only towards the end of the 19th century.

What is air made up of? Let us find the answer by preparing an invisible dish. You have to first get ready with the ingredients in the correct proportions. All the ingredients of this dish are invisible (gases)! This dish requires the following ingredients:

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Nitrogen (N) Oxygen (O) Argon (Ar) Carbon dioxide (CO2) Hydrogen (H) Ozone(O3) Helium (He)

78% 21% 0.9% 0.03% traces traces traces

Procedure

Pure dry air

Pour the ingredients into a glass jar. You do not need to stir or mix to get the invisible dish! Now add a trace of ozone. Ozone has a distinct smell. But the smell disappears soon. Why? You have just prepared air. Air is a mixture of colourless gases (generally in fixed proportions). The dish you have prepared is pure dry air. It is different from the air around us. Let us find out how.

Finding the missing ingredients Fill a metal container with water. Add 5-6 ice cubes to it. Wait for a few minutes. Water droplets appear on the outer surface of the container. Where did these water droplets come from? They came from the air around us. Water is present in the form of water vapour in the air. It is one of the missing ingredients in pure dry air. Why do you have to add ice cubes to water for the water droplets to appear on the outer surface of the container?

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Water vapour or humidity The amount of water vapour in air depends upon how hot the air is. Hot air can hold more water vapour. The amount of water vapour in air is not constant. It keeps changing. However, at any given temperature, a fixed volume of air can hold only a fixed amount of water. This is called maximum humidity.

Where does the water vapour come from? Sources of water vapour in air are:

Oceans

Lakes

Plants

Glaciers

Rivers

Wet soil

Snow

Energy from the sun is continuously changing water to water vapour.

Fickle water vapour Water vapour has been present in the air since the birth of the planet. Almost all the water vapour is present in the lower layers of the air around us. The content of water vapour is not fixed. It varies from 0.5 to 4 % in a given volume of air. The water vapour content in the air around us depends upon these factors:

Location

Elevation of the place

Season

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Which is heavier - soggy air or dry air? Ask your friend this question: which is heavier - humid air or dry air? Your friend is most likely to laugh at the obvious answer and reply humid air, of course! Your friend is wrong. Let us find out why. Humid air contains a high percentage of water vapour (H2O). The water vapour displaces an equal volume of dry air. Dry air contains 78% 96% nitrogen , 21% oxygen and Heavy 1% of other gases. They are Dry air gases heavier. Humid air with 4% Nitrogen - 78% water vapour contains only Oxygen - 21% 96% of the heavier gases. Others - 1% Humid air What do you conclude? On a cold, clear winter day is the air humid or dry?

Is air humid or dry during winter? During winter, land loses its absorbed heat slowly. As a result the temperature of the layers of air in contact with the earth, becomes low. As a result, the air is cold. Cold air cannot hold much moisture. Humidity of cold air is low. Therefore, the air is dry during winter.

Naturally curly hair becomes more curly in the rainy season and breaks easily in winter Too much humidity or too little humidity affects our daily lives in many ways. High humidity damages anything that can absorb the moisture in air. It is difficult to close doors during the rainy season. Very low humidity damages our skin and our hair breaks more easily. Why do medicine bottles have cotton wool packing?

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Why is it difficult to close doors in rainy season? During rainy season, the relative humidity of air is high. Wooden doors absorb the moisture in air and expand slightly or get warped. As a result, they do not fit into the door frames.

The other missing ingredient The stream of fine particles dancing in the air are the dust particles in it. Air around us has microscopic particles floating around. Water vapour present in air condenses on these particles to form clouds. What would happen if we removed all the microscopic particles from the air around us?

Condensation Water vapour in air is invisible. Under certain conditions it becomes visible in a variety of ways and forms. But, for condensation to occur, the temperature of air must be lower than its dew point. The air can be cooled by  loss of heat (radiation).  contact with colder surface.  mixing with cold air.  expanding of air as air current rises.

How do microscopic particles enter the air around us?

Fine dust from soil,

Pollen,

Smoke,

Chemical salts from evaporation of saline water.

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Traces of these microscopic particles of chemical salts in the upper layers of air are important for life on earth. 99% of the water vapour and almost all the dust particles are present in the layers of air close to the earth.

Pure dry air to air around us By adding water vapour and dust particles to the invisible dish, you have now prepared the air around us. The air around us is like a shell enveloping the lands and the oceans of the earth. But you cannot store your dish. This air spreads above the surface of the land and the oceans. This is called the atmosphere. The air enveloping the surface of the earth or the atmosphere reaches a height of at least 960 km. Beyond this height, it gradually merges with the empty space. How do we know this?

960 km

The Atmosphere The atmosphere is compared to a huge ocean of air. Everything on our planet exists underneath this invisible ocean. The atmosphere has 5 distinct layers. Most water vapour and carbon dioxide is found within 12 km of the atmosphere. The minute quantity of ozone that is present in air is found at about 20 km in the atmosphere. Towards its upper limits, the atmosphere is very thin.

We have gained knowledge of the extent of the atmosphere by direct and indirect observations. Sources of direct observation are:  Upto a height of 8.8 km by mountaineers.  Upto 32 km by hot air balloons. Data from unmanned weather balloons and weather satellites,  Upto ~45 km by balloons carrying instruments.  From satellites carrying instruments.

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Sources of indirect observation are:  Aurora - gives information about the gases present at great heights.  Meteors - give information about upper air temperature.  Sound waves, radio waves, radar and TV waves give us information about the electrical properties. Scientists have fixed 960 km as the approximate upper limit of the atmosphere. Aurora Borealis or the Northern light is the basis for this conclusion.

Composition of air: then and now The composition of air has undergone a remarkable change since the birth of our planet ~4.6 billion years ago. The gases in the atmosphere of the infant earth were mostly methane, ammonia and water vapour. How different these ingredients are from those of the invisible dish you prepared! There was no free oxygen or nitrogen in the primitive atmosphere. How and why did the composition of air change?

Evolution of the atmosphere Life on earth first appeared around 3.5 billion years ago. The early primitive life forms did not need air or oxygen.* When ferns appeared on earth, oxygen was introduced into the atmosphere. This brought about a dramatic change in the composition of the atmosphere. Oxygen changed methane to carbon dioxide and water and ammonia to nitrogen and water. Evolution of plants and animals helped to stabilize the atmosphere. The composition of the atmosphere has remained more or less the same for the last 2 billion years. *Such life forms are called anaerobic life forms. These life forms exist even today.

Is the air the same everywhere? Pure air is a perfect mixture of the gases present in it. Its composition remains almost the same all over the world upto a height of 32 km.

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Nitrogen 78% Oxygen 21% Argon 0.9% Carbon dioxide 0.03% Hydrogen traces Ozone traces Helium traces However, the percentages of carbon dioxide and water vapour are not the same everywhere.

Carbon dioxide level in air The carbon dioxide content has changed most during our planet’s long history. At present, the average level of carbon dioxide is ~0.03 %. The percentage of carbon dioxide is marginally greater near

volcanoes,

factories,

villages

and forest fires

Increase in the level of carbon dioxide is dangerous.

What would happen if the carbon dioxide level increases even marginally? Even a small increase in the level of carbon dioxide in air is harmful, as it results in  global warming.  melting of ice caps.  rise in the levels of oceans and seas.  destruction of many forests.

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12.2 Properties of air Half full or half empty? Some people may say that this glass is half empty and some others may say that it is half full. Both are wrong. The glass is actually full. Are you surprised? What appears as an empty glass is actually filled with air. This apparently half-empty glass is a full glass - half filled with air, half filled with water. When air is completely removed, a vacuum is created.

Vacuum Vacuum means empty space in Latin. Vacuum does not contain even air. Otto Von Guericke conducted an interesting experiment in 1640 to demonstrate the effect of atmospheric pressure when air is removed from a container. He took two tight fitting hemispheres. The hemispheres had hooks on the sides. Air present in the space between the hemispheres was removed. Even two teams of eight horses could not pull the hemispheres apart as the atmospheric pressure pressed on the vacuum inside the hemispheres.

Air occupies space Children like doing these:

inflating a balloon blowing bubble gum

blowing soap bubbles

What is common to all these? Air occupies the space inside a balloon, a bubble of the bubble gum or a soap bubble. Why do we blow air when two pages are stuck together?

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The invisible air has weight We cannot find the weight of air in an empty glass. Then how can we prove that air has weight? Even in an empty balloon there will be some air. Take a basketball. Remove all air from the basketball by pressing it on a flat surface. Weigh the basketball and note down its weight. Now, blow the basketball until it is half-filled with air. Weigh it and note down the weight. Blow the basketball until it is fully inflated. Again weigh it and note down the weight. Compare the weights of the empty ball, half inflated ball and the fully inflated ball. What does this prove?

Air exerts pressure Air envelops the earth. It rests on the outer surface of the earth. Air presses down on everything on the earth’s surface. This is called air pressure. How is air pressure caused? There are millions of molecules of various gases in air. These molecules of gases move around at great speeds in all directions. They bump into and bounce off each other while they are moving. This movement and weight of the gases create pressure.

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How to measure air pressure Air pressure is caused by its weight. Measuring air pressure is like weighing a 960 km high column of air in a balance! Torricelli was the first to device a simple method to measure the air pressure. He took a long glass tube open at one end and filled it fully with mercury. Closing the open end of the glass tube with his thumb, he inverted it into a glass dish containing mercury. When his thumb was removed, the mercury dropped to 76 cm level. The length of the tube or its cross section did not matter. The vacuum above the mercury column is still called “Torricelli’s vacuum”. Modern barometers are based on this principle.

Why don’t we feel the air pressure? Air does not exert pressure only downwards or upwards. Air exerts pressure equally in all directions and on all surfaces. This pressure is exerted on all parts of our body. Our body, in turn, exerts the same pressure in all directions but outwards. If the air pressure was greater, we would be crushed. What would happen if we removed all the air from an empty can?

Why do mountaineers suffer from a bleeding nose at high altitudes? Air pressure decreases with height. As a mountaineer climbs higher, the air pressure decreases. But the body pressure acting outwards does not decrease. Therefore, the pressure exerted by the mountaineer’s body is greater than the air pressure. This causes the capillaries in the nose to bleed.

Distribution of air pressure Upto 99% of the air in the atmosphere is within 32 km height from the earth’s surface. Half of the entire weight of air is below 5.5 km from the earth’s surface. Air pressure is greatest at the surface and decreases with height.

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Why do mountain climbers carry oxygen? Air is very thin (or spread out) at greater heights. It has fewer molecules of the various gases including oxygen. Each time a mountain climber breathes in, he/she gets less oxygen. Hence, he/she feels breathless. At great heights, even walking a few steps feels as if one is climbing a mountain.

Air can be compressed Invisible air has strength when compressed. Compressed air is used  to make wheels move faster, better and bear more weight.  in mines to lift liquid substances.  in machines for digging.  in the brake system of trains and some automobiles.

How does the air get heated? The sun supplies the energy to heat air in the atmosphere as well as the earth’s surface. The heat energy coming from the sun heats the air both directly and indirectly. Directly: when the upper layers of the air absorb the heat energy from the sun as the sun’s rays pass through. Indirectly: when the lowest layer in contact with the earth’s surface gets heated by heat given out by land and water surfaces.

When do the sun’s rays produce heat? Sun’s rays do not always produce heat. Let us see what happens to the sun’s rays in their journey from the sun to the earth’s surface. The sun’s rays  pass through glass, or the air of the atmosphere without loss of energy.

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

are reflected by smooth, shiny surfaces like water and snow.  are absorbed by land and water surfaces. Energy in the rays is turned into heat only when they are absorbed. Solids absorb better than liquids; liquids absorb better than gases.

Why do swimmers and skiers get sunburnt easily? Water in the swimming pool and snow on the ski slopes reflect sun’s rays. At the same time, the swimmers and skiers absorb the sun’s rays that strike them. Therefore, they get a double dose of energy from the sun.

Example of what happens to the sun’s rays in the solarium in the picture given below Sun’s rays pass through the glass. The glass remains cold to touch. The mirror reflects the sun’s rays. The mirror also remains cool. The man’s face becomes warm or hot. Why do the glass and mirror remain cool while the man’s face becomes warm or hot? The glass transmits the sun’s rays. The mirror reflects the sun’s rays. The man’s body absorbs the sun’s rays.

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12.3 Air and life on earth Like all other planets, the earth also has an atmosphere But, the air enveloping the surface of the earth is unique because it can support a diversity of life on this planet. The various gases present in the air, whether in great quantity (like nitrogen) or in traces (like ozone and carbon dioxide), have important roles to play in making our home planet special in the universe. Let us examine the role of various gases present in the air around and above us.

Nitrogen This gas is essential for plant growth. However, plants cannot use it directly. Nature comes to the help of plants and converts nitrogen in the air to ammonia by using an enzyme. Nature does this through the root nodules of leguminous plants (beans, groundnut etc.) Animals also require nitrogen. They get their nitrogen from plants. Man has also found a way of converting nitrogen present in the air to ammonia. The process for preparing ammonia is known as the Haber process. Under special conditions, nitrogen reacts with atmospheric oxygen. Lightning on a cloudy day provides the high energy required for nitrogen to react with atmospheric oxygen. These oxides of nitrogen dissolve in the water molecules present in the clouds and come down with the rain. Some of the nitrogen in the atmosphere finally gets converted to nitrates in the soil. Nitrogen: an important industrial raw material In the fertilizer industry, it is used to prepare ammonium sulfate, potassium nitrate and urea. It is used to prepare TNT (trinitrotoluene), nitroglycerine and nitrocellulose in the explosives industry. Nitrogen compounds are also used in the manufacture of dyes and medicines.

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Alfred Nobel Alfred Nobel (1833 - 1896) was a chemist and an engineer. He invented dynamite. According to his will, Nobel Prizes were instituted for medicine, physics, chemistry, economics, peace, literature. Nobel Prizes were awarded for the first time on December 10, 1910. The formal function of Nobel Prize has always been held on December 10 since that time. Nobel Prize winners are called Nobel laureates.

Oxygen: breath of life Life as we know it, is largely due to the oxygen we breathe. Even a few minutes without it, all life ends. Aquatic animals breathe the oxygen dissolved in water. The oxygen in water comes from the water plants. Oxygen is also needed for burning. Pure oxygen is stored in special cylinders and is given to seriously ill patients and asthma patients. It is carried by mountain climbers, astronauts and deep sea divers.

Minor gases - major role The 1% of the various minor gases present in the air have a major role to play. Of these gases, carbon dioxide and ozone are vital to life on earth. Plants take in carbon dioxide from air to prepare food. In this process, plants give out oxygen.

Oxygen

Carbon dioxide

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Ozone: the protector High up in the atmosphere, around 20 km from the surface of the earth, the ozone layer acts as a shield and protects the earth from the harmful UV rays present in the incoming solar radiation. But for this layer, all life on earth would have been scorched. Ozone is used to sterilize drinking water and to remove bad odours and tastes.

The restless air Which way does the air move? Upwards? Downwards? Sideways? The air around us is in constant motion. It blows across the earth’s surface. It also rises upwards from the earth’s surface and sinks towards the earth’s surface as well. The horizontal movement is called wind and the vertical movement is called air current.

Wind Wind travels under different names and has different identities. Calm does not disturb even a column of smoke. A breeze moves leaves and makes kites fly.

Calm

Breeze

A Zephyr moves gently. A gale or hurricane blows with great speed.

Zephyr

Gale

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Wind has many more faces and many more names. What makes air so restless?

Sun: the puppeteer and the puppet air It is the sun that makes the wind blow hot or blow cold, blow gently or blow violently. The sun pulls the strings by heating the earth’s surface unequally. Places which receive direct rays are hotter. Land surfaces are hotter than water bodies. Even on land, rocks and dark, dry soil get heated more than wet, light-coloured soil. Grasslands are hotter than forests. The difference in temperature results in the difference in air pressure (Higher the temperature lower is the pressure and vice versa). How does this make the wind blow?

Pressure and wind The wind always blows from a high pressure area to a low pressure area. The strength of the wind Low pressure area depends on the pressure differences. When the High pressure area pressure difference is  more, winds are strong.  small, winds are gentle or weak.  very small, there may be no wind at all.

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Temperature and pressure make the wind blow Generally, wind speeds are greater  at higher altitudes, as there is less effect of friction between the air and the land.  during winter than during summer as there is a greater pressure gradient.  over water than over land as the water surface is smoother and there are no obstacles in the path of the wind.

What would happen if the sun heated the earth’s surface equally everywhere? Air pressure is inversely proportional to air temperature. The sun’s heat energy does not heat all places on the earth’s surface equally. Even in a small area, there will be temperature differences and therefore pressure differences. Winds blow because of this pressure difference. If all the places on the earth including water surfaces were heated equally there would be no wind. It would be still! Can you think of the consequences? The equatorial region is heated almost equally everywhere. That is why there is generally no wind there.

How are winds named? Winds are named after the direction from which they blow. North East winds South West winds

Windward

Leeward

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Air: the moving force around us Throughout history, wind and air currents have been used in a variety of ways.

Wind dries clothes and seeds faster. Wind helps to separate grain from husk. Wind is a source of renewable, clean energy.

Wind provides the power for sailing. Wind helps birds and insects to fly. Wind helps in the dispersal of seeds. Windmill was used to run flour mills and draw water.

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12.4 Weather Elements of weather Once there was a man called Weatherman. His job was to tell the people the kind of day they could expect. He would go outside and hold out his hand and if it became wet, he would announce that there was rain on the way and recommend taking an umbrella. He would stand outside for a few minutes looking at the sky and announce that it would be a cloudy day and warm. If his coat flapped, he would announce a windy day and since he always had his coat on, he never predicted a cold day. Though everyone listened to him, there was a serious problem with his predictions - most of the time he was wrong! People thought hard and decided to understand weather better.

Daily weather bulletin Temperature, air pressure, humidity, precipitation, cloud cover and winds are the main elements of weather. A change in any one of these elements brings about a change in the weather. Let us find out what causes the changes.

Precipitation Precipitation represents the liquid and solid particles from the clouds that fall to the ground. Drizzle, rain, snow, sleet, hailstones, ice crystals are all different forms of precipitation. The main difference between the particles of water in the clouds and precipitation is in the size of the particles.

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The mass of an average raindrop is 1,00,000 times more than the mass of a water droplet in the cloud. Condensation alone, cannot produce precipitation. That is why all clouds do not bring rain.

Why do the elements of weather change? The sun heats the earth but not uniformly. As a result, some areas of the earth are hotter than the others. The sun moves the water from oceans, rivers, lakes and any wet surface into air as water vapour. The sun is the cause for winds and air currents. In fact, the energy from the sun is the source of all the changes in the weather.

Temperature and weather During daytime, the rays from the sun heat  the land surface.  the water on the earth’s surface.  and the air in the atmosphere. Even though the sun shines with equal intensity on land, water and the air in the atmosphere, they do not get heated equally. Land surface heats faster and cools faster than water surface. How does the difference in heating and cooling between land and water affect weather? The air over land gets heated faster during daytime and cools faster once the sun sets. Also, the air gets heated gradually till afternoon and then starts to cool.

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The sun and the magic of disappearing water There was a shallow pond and a large lake near my house. During winter, the water in the shallow pond came up to my ankles and water in the lake came up to my knees. During summer, the pond was completely dry and the water in the lake came up to my ankles. I was puzzled by this. Where did the water from the pond go during summer? Why did the water from the lake not disappear?

Energy from the sun and evaporation The energy from the sun changes water to water vapour just like water changes to steam when we boil water. If a small quantity of water is boiled for a long time, all the water changes to steam or water vapour. This is what happened to the water in the pond. As the lake had more water, only part of it changed to water vapour. The water vapour enters the air. The amount of water vapour in the air is called humidity.

Back to water Let us find out what happens to the water vapour that goes into the air. As the air rises up, it cools. The water vapour in the air again changes to droplets of water. The change of water vapour to water is called condensation.

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Different forms of condensation are:

Clouds

Dew

Fog

Mist

What would happen if the dust particles are not present in the air?

Role of dust particles in air Microscopic dust particles get into the atmosphere from the evaporation of salty water, from eruption of volcanoes as well as by human activity. Dust particles play a crucial role in the formation of clouds. Microscopic dust or fine particles of chemical salts attract water vapour particles i.e. they are hygroscopic. Water vapour in the air needs hygroscopic nuclei to condense and form clouds when the air temperature falls below dew point. If there were no microscopic dust particles in the atmosphere, there would be no clouds, no precipitation and no water cycle.

What goes up comes down As more and more water vapour condenses on the dust particles, the droplets of water in the clouds become heavy. They eventually fall to the earth. This is the process of precipitation. Different forms of precipitation are:

Snow Rain

Sleet Hail

The type of precipitation tells us about the temperature of the layers of air through which it is passing. Why don’t we see snowfall in most of India even during winter?

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12.5 Weather Forecasting Science of weather forecasting Now, our friend the weatherman is called a “meteorologist”. He realises that predicting weather is a scientific problem. How does the meteorologist predict the weather now?

How is weather forecast? The meteorologist gathers information about temperature, air pressure, wind speed wind direction, relative humidity. the condition of the sky, the nature and type of precipitation.

How does a meteorologist collect information? The meteorologist is not a lone observer of the weather conditions. There are hundreds of weather stations from where the information is collected. Some are

in the plains (in the cities).

on mountain tops.

at the top of high buildings.

Modern methods of gathering data Weather observations are made from the following:

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Weather balloons

Weather satellite

Weather ships

Radars

Radiosonde It has a thermometer, barometer, hygrometer and radio transmitter. Radiosonde is carried upto a height of ~ 22 km by a helium filled balloon. The radio transmitter automatically sends back information of temperature, pressure, relative humidity of the air through which it is passing. A radio receiver at a weather station records the signals emitted by Radiosonde the radiosonde.

What are the instruments used to gather information? Earlier our weatherman predicted weather based on what he saw and felt. Now he uses different instruments to collect data. Temperature is measured with a thermometer, a thermograph, a maximum and minimum thermometer. Air pressure is measured by a barograph, aneroid barometer, mercury barometer Relative humidity is measured by a psychrometer

Thermometer It is the instrument used to measure air temperature. Two types of thermometers are used to measure air temperature. They are:  liquid thermometers using mercury or alcohol as the expanding liquids. Both these are based on the principle that liquids expand whenever there is a rise in temperature. Alcohol thermometer is usually coloured red or blue, as alcohol is a colourless liquid.  metal thermometers similar to the ones used in a oven. Doctors use mercury thermometer to take the temperature of patients.

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Thermograph In this instrument, temperature is recorded on a graph paper mounted on a cylinder. The cylinder turns at a uniform speed and the pen records the temperature continuously. A special ink that does not freeze or evaporate is used in the thermograph. However, often its readings are not accurate.

Aneroid barometer It is the most commonly used barometer. Aneroid means without liquid. The most important part in the aneroid barometer is a small shallow metal can. Almost all the air is pumped out of it. Hence it is sensitive to air pressure. It is supported by a spring to prevent it from collapsing. Barograph is actually an aneroid barometer with a pen or a stylus instead of a pointer. It records the pressure on a rotating graph paper. Mercury barometer gives the most accurate measurement of air pressure. But it is difficult to carry around.

Relative humidity Relative humidity is the amount of water vapour in the air or absolute humidity compared with the maximum amount of water vapour that the air can hold or capacity of the air at a particular temperature. It is expressed as a percentage. Relative humidity =

Amount of water vapour present in a given volume of air x 100 capacity or the maximum water vapour it can hold

Psychrometer Psychrometer is used to measure relative humidity. It is based on the principle that evaporation, being an exothermic reaction, results in cooling. Dry air causes more evaporation than moist air. Psychrometer is also called a wet and dry thermometer A slim psychrometer has a handle and a chain. It can be whirled around to get more accurate readings.

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Wind vane Wind vane is the instrument used to determine the direction in which wind is blowing. Wind vane points into the wind or the direction from which the wind is blowing. For example, if the wind vane points towards the north east, it indicates a north east wind. It has a light arrow where the arrow tail points in the opposite direction. In airports, wind socks are used.

Anemometer Wind speed is measured by a anemometer. Wind blows at different speeds at different times. Wind speed near the earth’s surface is measured by the anemometer. Anemometer consists of cups that are hollow hemispheres. The cups face the same direction irrespective of the wind speed. They catch the wind on their open sides. The cups rotate and the number of rotations are marked on a scale.

Measuring snowfall Precipitation can be in the form of rain or snow. Rain is measured by a rain gauge. The amount of snowfall can be measured in two ways: 1. Measuring the depth of snowfall by using a scale. 2. Collecting snow in a rain gauge and allowing it to melt. Then measuring the snowmelt. Normally, 10 inches of snow is the same as 1 inch of rain.

Conditions of the sky: Clear or cloudy? Have you watched clouds in the sky? They are always on the move. Meteorologists are great cloud watchers. Clouds can be

thin like a feather. white or coloured.

large and billowy.

The colour and nature of clouds give the meteorologist valuable information.

layered like a cake.

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How does the weatherman give his weather report? He uses symbols to represent the various elements of weather. Station Model Sky cover Temperature (0C)

Rain S

R Snow

Wind direction Dew Point Wind speed (0C)

Sky cover is represented by

Clear

Partly cloudy

Cloudy

Weather and climate Over the radio, television and in newspapers, we get the daily weather report or the prediction of weather, at best for the next two days. Why is it that we do not get a daily climate report? Elements of weather and climate are the same.

Climate Weather is the daily status of temperature, humidity, precipitation, cloud cover and wind. Climate is the average of these conditions over a number of years. Weather is a daily happening. Climate describes the conditions that remain predictable. Weather changes day to day. Climate is the same for a place. Climate is affected by the same factors.

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12.6 Air pollution What is air pollution? Many harmful gases and minute solid particles get added to the lower layers of the atmosphere. The addition of harmful substances to the air causes air pollution.

Changing atmosphere Human activities and the atmosphere The composition of the atmosphere remained more or less the same for millions of years. But the Industrial Revolution marked the beginning of man’s activities introducing slow but definite changes. In the last fifty years or so changes in the levels of minor gases in the atmosphere have become alarming. Industries and our ways of living directly affect the air around us.

What are we doing to the air around us? We are adding dangerous amounts of harmful substances by our activities. Harmful substances added to the air around us are: Carbon dioxide from exhaust fumes of cars, lorries, two-wheelers, buses. Smoke from the burning of firewood. Carbon monoxide from incomplete burning of fuels. Smoke, microscopic particles, ashes from factories. CFC from refrigerators etc. Oxides of sulfur, nitrogen from refineries and power plants.

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How does the addition of carbon dioxide and carbon monoxide affect life on earth? Increase in the levels of carbon dioxide results in “global warming”. This in turn can cause  melting of polar ice.  consequent rising of the sea level.  floods and droughts.  destruction of the temperate forests.

Greenhouse effect The term greenhouse effect was used for the first time by the French mathematician Jean Fourier. He compared the role of carbon dioxide in the atmosphere to the glass roof of a “hot house” or “green house” used to raise tropical plants in cool climates. Carbon dioxide, like the glass roof, lets the short wave solar heat energy to pass through and heat the earth’s surface. Carbon dioxide does not let the heat the earth loses, to pass through it. The heat coming in and going out are trapped in the atmosphere’s lower layers.

Smoke from factories cause smog (smoke and fog) Air pollution due to smog is common in industrial areas and urban areas. Sometimes due to certain conditions in the atmosphere, fog does not clear quickly. The exhaust smoke from automobiles and factories mixes with the fog. A thick layer of fog and smoke or smog results.

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Smog causes  reduced visibility  irritation in the eyes and

respiratory system.

Acid Rain Oxides of nitrogen and sulfur occur in the gaseous state in the atmosphere. These get dissolved in rain water and come down as acid rain. Acid rain causes corrosion of some metals, erosion of marble (limestone), mortar etc. It is responsible for the destruction of softwood forests and for the pollution of ground water.

Comforts on earth and danger from above Refrigerators, air conditioners and air sprays make our lives comfortable. Unfortunately, they are dangerous if they reach the upper layers of the air. Many of these machines use a gas (CFC) for cooling. When this gas reaches the ozone layer, it destroys the layer. The harmful ultraviolet rays coming from the sun can, therefore, reach the earth. Long term exposure to the ultraviolet rays can cause skin cancer.

CFC - the killer coolant CFCs or chlorofluorocarbons are used in air conditioners, refrigerators and air sprays as a coolant. This gaseous compound when released into the atmosphere is carried to the upper layers or the stratosphere of the atmosphere by air currents. Here, they interact with ozone in an irreversible manner. It stays in the atmosphere for a number of years causing great damage. As it depletes the ozone layer, we are exposed to the dangerous ultraviolet rays.

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Protecting the ozone layer from further damage Ozone depletion is a major problem. While it may not be possible to “close the ozone holes”, we can certainly minimize further damage. An alternative to CFC must be used in refrigerators, air conditioners, air sprays.

Prevent pollution or perish How can we help to prevent further air pollution? The first step is to prevent too much carbon dioxide and carbon monoxide entering the air around us. We can do this by  fitting vehicles with gadgets to purify the exhaust fumes.  using unleaded petrol.  developing alternate fuels to run vehicles.  having enough ventilators and chimneys in houses.  using solar cookers and water heaters. We must spread the awareness among people about the increasing air pollution due to our activities.

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13.0 All About Water

Objectives Water surrounds us just as air. Water is the main constituent of most living species, including ourselves. We shall look at various aspects of water, including the water cycle and properties of water. We also examine pollution, purification and conservation of this important ingredient of life.

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13.1 Water: the most abundant substance on earth All life on earth depends upon water. Without water, plants, animals, birds, insects and human beings will wither and die.

Water has been present since the birth of our planet In the beginning, water was not found as snow on the mountains, as water in the rivers, or as water in the oceans. It was found mostly as water vapour in the atmosphere. The world of water may appear to us as in any one of the pictures below.

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View from space Our view of water on earth was incorrect until our journey into space. Our journey into space gave us the correct view of water on earth. The water on earth (above the surface, under the surface as well as in the oceans) is called the hydrosphere.

How is water distributed on earth? Only 2.7% of water in the hydrosphere is fresh water. This is found in snow, glaciers, lakes, rivers and clouds.

Snow

Seas

Glaciers

Lakes

Rivers

The remaining 97.3% is found in the seas and oceans. However, water is not evenly distributed either on land or in the oceans.

Clouds

Oceans

Distribution of water on land (land water) Water is not distributed evenly over the surface of the earth. The total amount of water on land is ~ 2.7% of the total water on earth (7.3% of the total area) Of this 2.7% ~ 2.04% is locked in glaciers and ice caps. ~ 0.61% is underground water (upto 400 m depth). ~ 0.009% is in the lakes of the world. ~ 0.001 is in the atmosphere as water vapour. ~ 0.0001% is in the major and minor rivers of the world.

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Distribution of water in the oceans If we look at our planet from the north pole, we can see that all the oceans are connected and the continents appear as islands in this massive ocean. In fact, the Babylonians thought that the world was a great mountain rising out of a great expanse of water. The Greeks believed that the oceans (Okeans), a powerful stream, surrounded the disc (the world). The ocean was considered to be the bond between heaven and earth. For our convenience, we divide this great expanse of water into five oceans - three with water in the liquid state and two with water as frozen ice. The Pacific Ocean is the largest ocean and the Arctic Ocean is the smallest. The Indian Ocean is landlocked in the north and the Antarctic has no fixed boundaries. If all the water on earth is evenly spread on the surface of the earth, it will cover the entire earth to a depth of ~ 3.2 kms!

How much of water on earth can be used by humans? 99.7% of the total volume of water cannot be used by us. Much of even the 0.3% of usable water is beyond reach ordinarily as it is mostly underground. Rivers are the main source of usable water. Yet, it is only 0.0001% of the fresh water. It is no wonder that rivers have been worshipped in almost all the cultures.

Water vapour

rs ie c la G

Seas

Oceans

Oceans and us Water in the oceans may not be directly useful to plants and animals on earth. But the oceans are very important to us. Oceans influence climate. They are a source of food, are the storehouse of minerals, are inexhaustible energy sources and are crucial for world trade.

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How do oceans influence the climate? Oceans influence the climate in a variety of ways. They influence the distribution of temperature and humidity. Oceans moderate climate as water heats more slowly and cools more slowly than land. They are called the “savings bank of solar energy”. Winds blowing over the oceans, or maritime winds, bring a lot of rain. Oceans also control the distribution of pressure and the prevailing winds. Evaporation of water from ocean surfaces is an important factor in the water cycle. Ocean currents - warm currents and cold currents - influence the climate of the coasts along which they flow. Oceans regulate and stabilize the climate of the earth.

Oceans as a source of food Oceans are a great source for a variety of foods. Both the aquatic animals and aquatic plants are easily available and almost inexhaustible. There are many varieties of edible fish, crustaceans, edible seaweeds and blue green algae. Blue green algae is a rich source of proteins.

Oceans are the storehouse of minerals Oceans are the storehouses of a variety of chemical salts and minerals. Apart from dissolved metal salts like chlorides and sulfates, rich reservoirs of petroleum and natural gas are also found in the oceans. Gold, platinum, diamonds and sand are also found in oceans.

Oceans as sources of non-conventional energy Water of the oceans is in constant motion. This motion generates energy. Attempts are being made to harness the energy of the waves and ocean tides. Tidal energy is an alternative source of clean energy.

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Geothermal energy is another form of energy from the oceans. The difference in the temperature between the surface water and subsurface water generates energy.

Oceans, trade and transport Ocean transport has been used since many centuries by many nations to conduct trade between European countries and India and the far east. The fall of Constantinople in the first century AD made it necessary to find new oceanic routes. Oceans provide the cheapest and easiest means of transport. Modern international trade depends on ocean transport. Ocean transport has the advantage of not requiring infrastructure. The North Atlantic Ocean route between North America and Europe is the busiest.

Water and living beings In all forms of life, water contributes most to body weight. 70% of the human body is water. 80% of the elephant’s body weight is water. 60% of most trees is water. 90% of a number of fruits and vegetables is water. Water content in some common food articles

Cucumber ~95%

Slice of bread ~25%

Potato ~75%

Butter ~15%

Mushroom ~92%

Egg ~73%

Milk ~87%

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The percentage of water in different parts of you and me Between 60 - 70% of our body weight is due to water. Let us see how much of it is in different parts.

Brain ~ 70% water

Blood ~ 82% water

Lungs ~ 90% water

Body cells ~ mostly water

Water is essential for all living beings Almost all the vital processes in humans, animals and birds as well as in plants are carried out in water medium. Apart from this, water is needed in a variety of ways.

The inexhaustible water Water has been used by all living beings ever since life began on earth. In fact, it is believed that life on earth started in water. Yet, the total amount of water on our planet has remained more or less the same. What is the secret of this? Water exists as water, ice and water vapour at the same time only on our planet. It has the unique ability to change from solid to liquid, liquid to solid, liquid to gas or gas to liquid. This unique property of water is responsible for the amount of water on earth not diminishing significantly.

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13.2 Nature’s gift: The water cycle The water cycle: the cycle with no beginning or end When did it begin? How does it operate?

Man has sought answers to these questions since ancient times.

Understanding the water cycle - early attempts From ancient times the water cycle has fascinated people. Homer, a famous Greek poet, wrote about it in 1000 BC. Anaxagoras (500 - 428 BC) of Greece was the first to give a “scientific” explanation. According to him, The sun ‘lifted’ water from the seas into the atmosphere. The water from the atmosphere, in turn, fell as rain. The rain then collected underground. This underground water fed the rivers. While Anaxagoras was not completely correct, he laid the basis for understanding the water cycle.

Scientific understanding of the water cycle During the 16th century, Leonardo da Vinci and others “observed” the water cycle. They understood the water cycle for what it was - a scientific phenomenon.

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The water cycle

Condensation Evaporation Surface run off

Precipitation

Water on earth Infiltration

Is the water cycle found on other planets also? For the water cycle to occur on a planet or any other member of the solar system, there must be plenty of water in the liquid state, appropriate amount of solar energy to convert water to water vapour, and enough hygroscopic (affinity to water) particles in the atmosphere. Above all, water must change from one state to another and exist simultaneously in all the three states. All these conditions are found only on the earth in the solar system.

Role of evaporation in the water cycle Evaporation can be regarded as the starting point of the phenomenon of the water cycle. Evaporation changes water from the liquid state to the gaseous state (water vapour). Evaporation takes place wherever there is sufficient heat energy and water source. However, in the water cycle, evaporation from the oceans and the seas is most crucial.

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How much water vapour is there in the atmosphere? The water vapour content in the atmosphere is only 0.001% of the total volume of water on earth. This minute amount is responsible for all the forms of precipitation. If we condense this amount of water vapour and uniformly spread it over the earth, it would form a ~ 2.5 cm thick layer! The content of water vapour in the atmosphere is not uniform. It varies from place to place. It also decreases with increase in the height of the atmosphere.

Is evaporation the same everywhere? Evaporation is affected by weather conditions like temperature (heat available), humidity, wind, and by the condition of water like its salinity, turbidity (clear or muddy), depth These factors have a combined effect.

Role of plants in the water cycle Plants contribute to the water vapour content in the atmosphere through transpiration. Transpiration is a complex process. During transpiration, the plant takes water from its roots. transports the water to all parts of the plant through the stem. retains just ~1% of the water taken from the soil for its use.

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gives back the rest of the water it takes from the soil as water vapour to the atmosphere through the leaves.

The invisible water vapour: where does it go? Water vapour is carried across the land and oceans by wind and vertically upwards by air currents. This upward movement of water vapour is important for the next stage of the water cycle.

Condensation: transition from gas to liquid This change of state of water vapour in the atmosphere to liquid water takes place in the upper layers of the atmosphere. Conditions necessary for condensation are air cooling below dew point (Dew point is not the temperature at which dew is formed), presence of microscopic salt particles that attract water. The change of water vapour in the atmosphere to liquid water can take place near the ground or in the upper layers of the atmosphere.

What is dew point? Let us understand this by a simple example. Take a 100 ml measuring glass. The capacity of the glass is 100 ml. Fill the glass with some water. Note the measure. This is the actual amount of water in the glass. Humidity or moisture in the air can be compared to this. Unlike the water in the glass, the capacity of the air changes with temperature. Often, at a particular temperature, the capacity will be equal to the actual amount of humidity. This temperature is called the dew point. This is also called the saturation point.

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Condensation near the ground Forms of condensation near the ground are: dew (dew point above 00C) white frost (dew point below 00C) fog (dew point above 00C) mist (dew point above 00C) Fog and mist are examples of condensation in the air near the ground. Clouds are examples of condensation in the upper layers of the atmosphere.

Formation of clouds All forms of condensation do not play a role in the water cycle. It is the many splendoured cloud that is vital. How are clouds formed? Clouds are formed when warm air rises, expands the volume of air increases and the air cools below dew point. This cooling process is different. the water vapour in the air condenses on microscopic salt particles in the air. Clouds formed when the dew point is above the freezing point are: Cumulus clouds and Stratus clouds. Clouds that are formed when the dew point is below the freezing point: Cirrus clouds Clouds are classified according to their shapes and height from the ground level. According to the shapes, clouds are identified as Cirrus clouds - high, thin, feathery clouds Cumulus clouds - having a flat base and rising domes Stratus clouds - layered clouds Clouds on the basis of height from the ground level are High clouds: 6,000 - 12,000 m Medium clouds: 2,000 - 6,000 m (also called alto clouds). Low clouds: ground level to 2,000 m (also called nimbus clouds). Generally, clouds are a modification, or a combination, of these groups. Meterologists have identified ten different groups in all.

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Adiabatic cooling Cooling of air and cloud formation Cooling of air takes place in two ways: (1) loss of heat (2) cooling due to expansion. In cooling due to expansion, hot air rises, expands and cools. Here, there is no loss of heat. The same amount of heat in that volume of air has to heat the increased volume of air. This is called adiabatic cooling. For the formation of clouds, adiabatic cooling is essential. This type of cooling occurs at increased heights.

Back to earth: precipitation This is the final leg of the never ending journey of water on earth. Not all the clouds are part of the precipitation process. When the condensed water droplets become too heavy, they fall down as rain when the air temperature is above 00C. snow when the air temperature is below 00C. sleet when the raindrops pass through layers of air below 00C. hailstorm.

Sleet During winter, sometimes the layer of air near the ground is below freezing. At the same time, the temperature of the upper layers is above 00C. When there is rain, the raindrops pass through the layer of air below freezing temperature before reaching the ground. The raindrops freeze and reach the ground as icicle needles. This is called sleet.

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Hailstones For the formation of hailstones, there must be: high temperature, a summer thunderstorm, hot air currents. Hailstorms occur in summer. The structure of a hailstone resembles the structure of an onion. It has an icy core and layers of snow and ice surrounding it. Hailstones can be as small as peas or as large as tennis balls.

The billion-year old sip of water The water cycle makes water available for life on earth.

Precipitation Condensation Water

Evaporation

Transpiration

In fact, the water you are having now may be billions of years old! Think about it!

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13.3 Properties of water What is water made up of? Water is a liquid. But it is made up of two gaseous elements - hydrogen and oxygen. Hydrogen and oxygen in water are present in the ratio of 2:1 That is, two parts (atoms) of hydrogen combine with one part (atom) of oxygen to form a molecule of water.

Can we prepare water in the laboratory? It is not possible to prepare water in the laboratory because it requires high temperature and the reaction is explosive.

Physical properties of water Water has no colour, taste, smell and is transparent only if it is pure. Absolutely pure water is rarely found in nature.

Many faces of water We know water as a liquid. But steam is also water. Ice is also water. Water changes to ice or snow at 00C and to steam at 1000C. At atmospheric pressure (i.e. 760 mm of mercury), freezing point of pure water is 00C and boiling point of pure water is 1000C

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Effect of pressure on boiling point of water The boiling point of water is not always 1000C. It changes with the change in pressure of air around us (atmospheric pressure). 760 mm of mercury is taken as the standard atmospheric pressure. Low pressure - lower boiling point High pressure - higher boiling point Can you give examples of this?

Density of water The density of a substance is the weight per unit volume of the substance. Density of a substance is also known as its specific gravity. The density of water is 1. It is used as the standard to find out the density of other substances. The density of sea water is 1.025. Is the density of ice the same as that of water?

Why is the density of water as a solid (ice or snow) less than water as a liquid? In ice, the water molecules form well-defined structures with some empty spaces or channels in them. This is why the density of ice is less than that of liquid water. In liquid, there are no such empty spaces. Therefore, ice floats on water.

Floating ice and fishes in the lake Fishes live in frozen lakes. How is it possible? When the temperature falls below 00C, the surface water of the lake begins to freeze. Floating ice does not easily allow this low temperature to reach the water below. Therefore, the temperature of the water inside the lake remains above the freezing point. The fishes in the lake survive even in an apparently frozen lake. During freezing winter, water pipes often burst, or develop leaks. Do you know why?

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Leaking or burst water pipes in winter In cold countries, during severe winter, the temperature often goes down below 00C. The water inside the pipes freezes and expands. The expanding ice presses against the walls of the pipes (as it occupies more space than liquid water). This puts pressure on the walls of the pipes. The walls of the pipes develop cracks or sometimes burst due to this pressure.

Chemical properties of water Water and the litmus test Take some water in a beaker. Dip a red litmus paper in it. What do you notice? There is no change in colour. Next, dip a piece of blue litmus in the same water. What do you notice? There is no change in colour. This shows that water is neutral. It is neither acidic nor basic. Litmus test is used to determine the acidic or basic nature of a substance or a water solution. If blue litmus turns red when dipped in a liquid, the liquid is an acid. There will be no change of colour if the liquid is a base (or water). If red litmus turns blue when dipped in a liquid, then the liquid is a base. There will be no change of colour if the liquid is an acid (or water).

Water and heat Heat some water in a vessel. After the water begins to boil, we can see steam rising from the surface of the water. Continue heating the water. Even after all the water has evaporated, water does not decompose into oxygen and hydrogen. Heating water even upto 1000C results only in a physical change. Water merely changes from the liquid state to the gaseous state.

Water and metals Water reacts with some metals at room temperature (for example, sodium, potassium and calcium) and with some metals when steam is passed over heated metals (example, magnesium).

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Reaction of water and sodium Take some water in a trough. Cut a small piece of sodium. Wipe the oil on the sodium piece with a filter paper. Drop the piece of sodium in water. The sodium piece darts around in water. Sometimes, a flame is seen near the surface of the water. Sodium metal reacts even with cold water to form a sodium hydroxide solution and hydrogen gas. Potassium and calcium also react with cold water at room temperature. Water and magnesium Take some cold water in a trough. Put a piece of magnesium ribbon in it. What do you notice? Unlike sodium, magnesium does not react with cold water. Take a clean and dry test tube or flask. Place a piece of wet glasswool in the test tube. Place a piece of magnesium ribbon in the test tube and heat it. What do you notice? The magnesium ribbon gradually begins to glow. A colourless gas is given out at the same time.

Action of moisture in the air on metals Moisture in the air reacts with some metals. Iron reacts with steam and with moisture and oxygen in the air around us. Leave some iron nails in a moist place for a few days. You will notice that the nails have a brown coating. We say the nails have rusted. Rust is nothing but iron oxide. The equation for this reaction is,

4Fe + 3O2 + H2O Water as a solvent Water can dissolve solids, liquids and gases.

2Fe2O3.H2O

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However, water cannot dissolve all solids, all liquids and all gases. Solutions with water as the solvent are called aqueous solutions. The symbol for this is “aq”.

Soda water has dissolved carbon dioxide To dissolve a greater quantity of carbon dioxide, pressure is applied while dissolving carbon dioxide in water. The soda bottles are then sealed tightly so that no gas escapes. When the seal is broken or the cap is removed, the pressure is released and carbon dioxide gas is released with a sound.

Water dissolves many gases Natural water has dissolved oxygen, nitrogen and carbon dioxide. Life on earth depends greatly on these dissolved gases. Dissolved oxygen is essential for marine life to survive. Dissolved nitrogen is converted to nitrates by certain plants. Dissolved carbon dioxide is used by marine plants to produce food.

What happens to the fish if the aquarium is filled with distilled water or boiled water? The fish often die and sink to the floor of the aquarium. Why? There is no oxygen in distilled or boiled water. How does a fish get oxygen? The gills of the fish are specially designed to take the dissolved oxygen from water. A fish takes water through its mouth and the water then passes over the gills. After the gills take the oxygen from the water, the water comes out through the gills.

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13.4 Salinity of water Where do we find salty water? It is not only in the seas and oceans that we find salty water. Many inland lakes, borewells and sometimes rivers have salty water. In the desert regions of Gujarat and Rajasthan, some of the wells and rivers have salty or brackish water.

How do salts get into water? The main causes of salt in water are rain, rivers and underground water.

Rain

Rivers

Underground water

Rain water dissolves salts present in rocks and soil. The dissolved salts then enters the rivers and underground water. The final destination of rivers and underground water is the ocean.

Why is sea water salty? Sea water is salty because of the salty water brought by the rivers and underground water and evaporation of sea water. The amount of dissolved salts or salinity in all the oceans and seas is not the same. The Atlantic Ocean has the highest average salinity and the Arctic Ocean has the least average salinity. How do we express salinity? Salinity is the amount of dissolved salts in one litre of water.

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What is average salinity? The salinity of ocean waters is not the same everywhere. The average salinity of ocean waters is 35%o. (35 g of dissolved salts in 1000 g of water.) Salinity of ocean waters depends upon the amount of evaporation, addition of fresh water from the rivers and rainfall and humidity. Waters of oceans/seas near the tropics have the highest salinity. Near the equator and the poles, the salinity is low. Salinity of inland seas and lakes is high. Why? Salinity of some inland lakes are given below. Lake Utah 220%o, Dead sea 240%o and Lake Van in Turkey 240%o

Do plants like saline water? Take two pots. Plant balsam plants in both of them. Water one of them with ordinary water and the other with saline water for a few days. What do you observe? The plant getting ordinary water grows well while the plant getting saline water wilts. Saline water is not good for agriculture.

Saline water and industries Saline water is not useful in industry. Saline water causes metals to rust and corrode. How can the dissolved salts be removed from saline water?

Experiment to purify salt water Half fill the distilling flask with salt solution and boil the solution. Collect the condensed vapour in the receiving flask. Taste the distilled water. When a solution is boiled, the water vapour escapes leaving behind the solid impurities. The vapour sent through a condenser is converted back to water. This process is called distillation of water. Distilled water has no odour, flavour or colour.

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Uses of distilled water Distilled water is used in car batteries (lead-acid battery) to dilute the acid. in the manufacture of some chemicals. in high pressure steam plants.

Why is it not good to drink distilled water? Distilled water is the “purest water”. You may, therefore, think that it is the best water to drink. But you are wrong! Distilled water is not good for us to drink as some essential salts required by our body are no longer present in it.

Which are the salts dissolved in sea water? Do you know how much of dissolved salts is present in sea water? Sea water has ~ 41 million tons of dissolved salts in every cubic kilometre of sea water! The dissolved salts are sodium chloride ~ 77.7 % magnesium chloride ~ 10.9 % magnesium sulfate ~ 4.7% calcium sulfate ~ 3.6% potassium sulfate ~ 2.5% Salinity varies, but the relative percentage of the major salts remains more or less the same. There is very little of dissolved calcium carbonate in sea water.

Why is calcium carbonate almost absent in sea water? The sea water/ocean water does not contain calcium salts in great quantities. Many of the aquatic animals with shells use the dissolved calcium salt in the sea water to build their protective shells.

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13.5 Water pollution Water pollution: A serious global problem Water can get polluted easily because solids can dissolve in it. liquids can mix with it. gases can be absorbed by it.

Water pollution - what are the reasons? Water is the most abundantly used substance. It is used

in homes for a variety of activities.

for agriculture.

in industries.

It is used by animals both to quench thirst and to keep cool. Water after use can contain many harmful substances. The contaminated water is let into rivers, rivers ponds lakes seas ponds, lakes and seas.

What is pollution? Our environment is capable of withstanding natural levels or concentrations of many harmful substances or forms of energy such as heat or sound. Pollution is the addition of these substances or energy forms to the environment at higher levels than it can handle. Pollution causing substances can affect air, water or land. Pollution may be local and short lived (such as noise pollution) or it can be widespread and long lasting (such as chemical pollution caused by fertilizers and insecticides).

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Sources of water pollution Water is polluted mostly by man’s industrial activities, life styles and personal habits.

Industries and water pollution Most industries use plenty of water at almost every stage of processing or manufacturing. Water let out by industries contains harmful chemicals and bacteria. Many of the industries let out untreated polluted water into rivers, lakes and tanks. These impurities mix with the water sources and pollute them. Apart from effluents, industries also release poisonous gases, industrial waste and compounds of metals. The gases released are carbon dioxide, oxides of sulphur, oxides of nitrogen. Industrial water comes from paint industries. dye industries. drug industries. Compounds of lead, mercury, cadmium and arsenic are also let into water sources. Water sources normally can cleanse themselves but cannot handle the addition of large quantities of poisonous substances.

Domestic and agricultural waste and water pollution In many countries, personal habits and agricultural practices are serious sources of water pollution. Sources of water pollution are:

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food waste

insecticides and pesticides

soaps and detergents

untreated sewage

fertilizers

animal and human waste

Crude oil spills and pollution of the oceans Since the 1960s huge oil tankers called “super tankers” are transporting crude oil across the oceans. Quite often accidents take place and the crude oil is spilt over the oceans. This causes great damage to marine life. The oil spill near Alaska (USA) destroyed marine life. Super tankers cause super spills which result in super pollution.

Oil spills and destruction of marine life The gulf war of 1991 is a good example of the destruction of marine life due to oil spills and air pollution due to oil refineries catching fire. Oil spills on ocean surfaces is harmful to aquatic life because it prevents sufficient amount of sunlight from penetrating the ocean water, reduces the level of dissolved oxygen. renders the feathers of birds and gills of fishes ineffective. Oil spills can catch fire and cause air pollution. Oil refineries catching fire, caused air pollution as well.

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Why is water pollution harmful? The ill effects of water pollution are more than the foul smell and the dirty appearance. The effect of water pollution is felt by all living organisms on earth. Polluted water has less dissolved oxygen. This results in less oxygen available to marine plants and animals.

Water pollution and diseases Polluted water has many harmful microorganisms. Drinking polluted water results in diseases like cholera, typhoid and dysentery.

What are microorganisms? The microorganisms are bacteria, fungi, protozoa, algae and viruses. They enter the human body, through the air we breathe, the water we drink or by coming into contact with an infected person or through insect bites. Disease causing bacteria can be either air or water borne. In developing countries, water borne bacterial diseases like typhoid, cholera, tuberculosis etc. are common.

Pollution due to metallic compounds Polluted water often contains dissolved metallic compounds. These soluble metallic compounds pollute even underground water. Arsenic and fluoride poisoning from the polluted underground water cause serious health problems. Fluoride poisoning results in dental decay, bone decay and deformity. Arsenic poisoning results in loss of hair, accumulation of arsenic in the body and slow death.

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It is time that we stop polluting our water sources Some of the measures we can take are monitoring the movement of oil tankers, treatment of sewage water, limited use of fertilizers, pesticides, chemical treatment of industrial waste and human waste and garbage management. Above all, creating awareness of the dangers of water pollution among the general public is essential.

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13.6 Purification of water Water for drinking The main source of drinking water is the rain water that falls on the ground. This water flows above the surface or seeps through the soil before joining a stream, a river or a lake. While animals drink water from any source, we cannot drink water which is not purified.

Purification of water Even pure rain water is not really pure. It has undissolved small particles, dissolved gases, dissolved chemical salts. Water, therefore, has to be purified before it can be used by us.

Why should we drink purified water? There are a number of harmful microbes and bacteria in the soil. The harmful microbes like typhoid and dysentery germs can contaminate rain water as it seeps through the soil. In addition there are harmful chemicals which dissolve in rain water. Drinking such water can cause serious illnesses.

Problems of making water fit to drink Water from rivers or lakes must be made free from sediments, odour and taste, living organisms and dissolved salts causing hardness. Drinking water must also be clear. Engineers, bacteriologists, chemists and even meteorologists have to work together to make water fit for drinking.

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From water source to taps Water from a lake, river or a pond has to travel a long way, before it reaches the taps at home.

Stages of purification Making water fit for drinking and cooking has two important stages. They are removal of undissolved impurities and removal of dissolved impurities.

Addition of chlorine and lime

Removal of undissolved impurities Water is first pumped from a river or a lake to a water tank. Alum and lime are added to the water in the tank. The small particles and suspended particles come together or coagulate. If the water is left undisturbed for some time, these particles settle down as sediment.

Removal of dirt water - Filtration Water is passed through layers of sand and gravel in a chamber. The dirt in the water gets collected on the sand and gravel. This is the process of filtration. Now the water is ready for the second stage of purification.

Laboratory Filtration A solution is prepared by dissolving a solid in a solvent. To obtain a clear solution, free from solid particles, the solution is filtered. The process is called filtration.

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The solution is poured into a cone of folded filter paper, which is supported in a glass funnel. The spaces between the paper fibres hold back the tiny solid particles and allow only the liquid to pass through it. The clear filtered solution is called the filtrate, the solid retained on the paper is called the residue.

Chemical treatment Chlorine is added to water to kill harmful microorganisms. Water is still not to ready to start its journey through the pipes. Now, lime is added to water to remove the remaining impurities. Water is now ready for use.

Purification of water in rural areas There are many areas where there are no taps. People use any available source of water to cook and to quench their thirst. They must purify water by boiling, filtering and adding potassium permanganate to water.

Uses of water Water is essential for life processes In human beings of all ages and sizes, in animals of all shapes, sizes and habitats and in plants big and small.

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Water and human body Most of the biochemical reactions in human beings takes place in water medium. Water is required for digestion, assimilation of food, transportation of nutrients and excretion of waste. Human blood is mostly water.

Water and plants Plants require water for many of their essential functions. Water is essential for germination of seeds. preparation of food. transportation of food.

Running water and energy Running water is a source of inexhaustible energy. It was used for a few centuries to run flour mills. to run simple machines. to move around. to transport logs. Discovery of steam power ushered in the industrial revolution. Hydel power is an important source of energy.

Water and industries Water is used in industries for various purposes. Some industries where water is used in large quantities are

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Paper industry

Textile industry

Chemical industry

Petroleum industry

Drug industry

Dye industry

Iron and steel industries

Water as a coolant Water is used as a coolant because of its high specific heat. The specific heat of water is 1.0 cal/g0C. It is used to cool engines of cars, buses and trucks.

Water as a means of transportation Water has been an inexpensive means of transportation for centuries. Even today water is used to transport logs. to transport heavy raw materials to factories. to transport finished products to ports. as a means of inland transportation.

Water and recreation Water is a wonderful source of recreation and sports.

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Conservation of water Diminishing water Fresh water, once considered inexhaustible, is slowly but surely becoming less plentiful. Large areas of Africa and Asia often witness water famines. Women walk many kilometres for water in many countries. And yet, in urban areas of most countries water is wasted. It is predicted that future wars will be fought over water.

Conserve or perish Conservation of water means prevention of wastage of water. How can we conserve water? Possible measures for the conservation of water are:

minimizing pollution

preventing cutting down of trees

preventing overgrazing

preventing surface run off

recycling waste water

harvesting rain water

reusing domestic waste water

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You too have a role to play Close the taps after use. Do not keep water running in the taps while you are doing other things. Do not use tap water for watering the plants. Do not waste tap water. It is purified water. Do not pollute rivers and lakes. Together we can make a difference.

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