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6, , Physical and, Chemical Changes, , E, , very day you come across many, changes in your surroundings., These changes may involve one, or more substances. For example, your, mother may ask you to dissolve sugar, in water to make a cold drink. Making a, sugar solution is a change. Similarly,, setting curd from milk is a change., Sometimes milk becomes sour. Souring, of milk is a change. Stretched rubber, band also represents a change., Make a list of ten changes you have, noticed around you., In this chapter we shall perform some, activities and study the nature of these, changes. Broadly, these changes are of, two kinds, physical and chemical., , 6.1 PHYSICAL CHANGES, Activity 6.1, Cut a piece of paper in four square, pieces. Cut each square piece further, into four square pieces. Lay these pieces, on the floor or a table so that the pieces, acquire the shape of the original piece, of paper (Fig. 6.1)., Obviously, you cannot join the pieces, back to make the original piece, but is, there a change in the property of the, paper?, , Activity 6.2, Collect the chalk dust lying on the floor, near the chalkboard in your classroom., Or, crush a small piece of chalk into, dust. Add a little water to the dust to, make a paste. Roll it into the shape of a, piece of chalk. Let it dry., Did you recover chalk from the, dust?, , Activity 6.3, , Fig. 6.1 Paper pieces, , Take some ice in a glass or plastic, tumbler. Melt a small portion of ice by, placing the tumbler in the sun. You have, now a mixture of ice and water. Now, place the tumbler in a freezing mixture, (ice plus common salt)., Does the water become solid ice once, again?, SCIENCE, , 58, , 2020-21

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Activity 6.4, Boil some water in a container. Do you, see the steam rising from the surface of, water? Hold an inverted pan by its, handle over the steam at some distance, from the boiling water. Observe the, inner surface of the pan., Do you see any droplet of water, there?, , Activity 6.5, CAUTION, Be careful while handling a flame., Hold a used hack-saw blade with a, pair of tongs. Keep the tip of the free, end of the blade on the gas stove. Wait, for a few minutes., Does the colour of the tip of the blade, change?, Remove the blade from the flame., Observe the tip once again after some, time., Does it get back its original colour?, In Activities 6.1 and 6.2 above, you, saw that paper and a piece of chalk, underwent changes in size. In Activities, , 6.3 and 6.4, water changed its state (from, solid to liquid, or from gas to liquid). In, Activity 6.5, the hack-saw blade, changed colour on heating., Properties such as shape, size, colour, and state of a substance are called its, physical properties. A change in which, a substance undergoes a change in its, physical properties is called a physical, change. A physical change is generally, reversible. In such a change no new, substance is formed., Let us now consider the other kind, of change., , 6.2 CHEMICAL CHANGE, A change with which you are quite, familiar is the rusting of iron. If you, leave a piece of iron in the open for some, time, it acquires a film of brownish, substance. This substance is called rust, and the process is called rusting, (Fig. 6.2). Iron gates of parks or, farmlands, iron benches kept in lawns, and gardens, almost every article of iron,, kept in the open gets rusted. At home, you must have seen shovels and spades, getting rusted when exposed to the, , Fig. 6.2 Rusting iron, PHYSICAL AND CHEMICAL CHANGES, , 59, , 2020-21

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atmosphere for some time. In the, kitchen, a wet iron pan (tawa) often gets, rusted if left in that state for some time., Rust is not iron. It is different from iron, on which it gets deposited., Let us consider a few more changes, where new substances are formed., , Activity 6.6, , light (Fig. 6.3). When it is completely, burnt it leaves behind a powdery ash., Does the ash look like the, magnesium ribbon?, The change can be represented by, the following equation:, Magnesium (Mg) + Oxygen (O2) →, Magnesium oxide (MgO), , (To be demonstrated by the teacher), , The equations here are different from, those in mathematics. In equations, of this kind, the arrow implies, ‘becomes’. No attempt should be made, to balance chemical equations at this, stage., , CAUTION, It is dangerous to look for long at the, burning magnesium ribbon. The, teachers should advise children not, to stare at the burning ribbon., Get a small piece of a thin strip or, ribbon of magnesium. Clean its tip with, sandpaper. Bring the tip near a candle, flame. It burns with a brilliant white, , Fig. 6.3 Magnesium ribbon burning, , Collect the ash and mix it with a, small amount of water. Stir the mixture, (aqueous solution) well. Test the mixture, with blue and red litmus papers., Does the mixture turn red litmus, blue?, Does the mixture turn blue litmus, red?, On the basis of this test, how do you, classify the aqueous solution — acidic, or basic?, On dissolving the ash in water it, forms a new substance. This change can, be written in the form of the following, equation:, Magnesium oxide (MgO) + Water, (H2O) → Magnesium hydroxide [Mg(OH)2], As you have already learnt in, Chapter 5, magnesium hydroxide is a base., So, magnesium oxide is a new substance, formed on burning of magnesium., Magnesium hydroxide is another new, SCIENCE, , 60, , 2020-21

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substance formed by mixing magnesium, oxide with water., , Activity 6.7, (To be demonstrated by the teacher), Dissolve about a teaspoonful of copper, sulphate (blue vitriol or neela thotha) in, about half a cup of water in a glass, tumbler or a beaker. Add a few drops of, dilute sulphuric acid to the solution., You should get a blue coloured solution., Save a small sample of the solution in a, test tube or a small glass bottle. Drop a, nail or a used shaving blade into the, remaining solution. Wait for half an, hour or so. Observe the colour of the, solution. Compare it with the colour of, the sample solution saved separately, (Fig. 6.4)., , colour of the solution from blue to, green is due to the formation of iron, sulphate, a new substance. The brown, deposit on the iron nail is copper,, another new substance. We can write, the reaction as:, Copper sulphate solution (blue) + Iron, → Iron sulphate solution (green), + Copper (brown deposit), , Activity 6.8, Take about a teaspoonful of vinegar in, a test tube. Add a pinch of baking soda, to it. You would hear a hissing sound, and see bubbles of a gas coming out., Pass this gas through freshly prepared, lime water as shown in Fig. 6.5., What happens to the lime water?, , Iron sulphate (greenish), Copper sulphate, (blue), , Fig. 6.4 Change in colour of the copper sulphate, solution due to reaction with iron, , Do you see any change in the colour, of the solution?, Take out the nail or the blade., Has it changed in any way?, The changes that you notice are, due to a reaction between copper, sulphate and iron. The change of, , The change in the test tube is as, follows:, Vinegar (Acetic acid) + Baking soda, (Sodium hydrogencarbonate) →, Carbon dioxide + other substances, The reaction between carbon dioxide, and lime water is as follows:, , PHYSICAL AND CHEMICAL CHANGES, , 61, , 2020-21

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Carbon, dioxide, , Vinegar +, Baking soda, Lime, water, , Fig. 6.5 Set up to pass gas through lime water, , Carbon dioxide (CO2) + Lime water, [Ca(OH) 2 ] → Calcium Carbonate, (CaCO3) + Water (H2O), When carbon dioxide is passed, through lime water, calcium carbonate, is formed, which makes lime water milky., The turning of lime water into milky is a, standard test of carbon dioxide. You will, use it in Chapter 10 to show that the air, we breathe out is rich in carbon dioxide., In Activities 6.6–6.8, you saw that, in each change one or more new, substances were formed. In Activity 6.6,, the ash was the new substance formed, when magnesium was burnt in air. In, Activity 6.7, the reaction of copper, sulphate with iron produced iron, sulphate and copper. Both of these are, new substances. Copper was deposited, on the shaving blade of iron. In Activity, 6.8, vinegar and baking soda together, produced carbon dioxide, which turned, lime water milky. Can you name the new, substance formed in this reaction?, , A change in which one or more, new substances are formed is called a, chemical change. A chemical change, is also called a chemical reaction., Chemical changes are very important, in our lives. All new substances are, formed as a result of chemical changes., For example, digestion of food in our, body, ripening of fruits, fermentation of, grapes, etc., happen due to series of, chemical changes. A medicine is the end, product of a chain of chemical reactions., Useful new materials, such as plastics, and detergents, are produced by, chemical reactions. Indeed, every new, material is discovered by studying, chemical changes., We have seen that one or more new, substances are produced in a chemical, change. In addition to new products,, the following may accompany a chemical, change:, n Heat, light or any other radiation, (ultraviolet, for example) may be given, off or absorbed., n Sound may be produced., n A change in smell may take place or, a new smell may be given off., n A colour change may take place ., n A gas may be formed., Let us look at some examples., You saw that burning of magnesium, ribbon is a chemical change. Burning, of coal, wood or leaves is also a chemical, change. In fact, burning of any, substance is a chemical change., Burning is always accompanied by, production of heat., SCIENCE, , 62, , 2020-21

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Explosion of a firework is a chemical, change. You know that such an, explosion produces heat, light, sound, and unpleasant gases that pollute the, atmosphere. That is why you are advised, not to play with fireworks., When food gets spoiled, it produces, a foul smell. Shall we call this change a, chemical change?, You must have noticed that a slice of, an apple acquires a brown colour if it is, not consumed immediately. If you have, not seen this change in colour, cut a, fresh slice of apple and keep it away for, some time. Repeat the same activity with, a slice of potato or brinjal. The change, of colour in these cases is due to the, formation of new substances. Are not, these changes chemical changes?, In Chapter 5, you neutralised an acid, with a base. Is neutralisation a chemical, change?, , A protective shield, You must have heard of the ozone layer, in our atmosphere. It protects us from, the harmful ultraviolet radiation which, come from the sun. Ozone absorbs this, radiation and breaks down to oxygen., Oxygen is different from ozone. Can, we call the breaking down of ozone a, chemical change?, If ultraviolet radiation were not, absorbed by ozone, it would reach the, earth’s surface and cause harm to us, and other life forms. Ozone acts as a, natural shield against this radiation., , We learnt in Chapter 1 that, plants produce their food by, a process called photosynthesis., Can we call photosynthesis, a chemical change?, Paheli said that even digestion, is a chemical change., , 6.3 RUSTING, , OF, , IRON, , Let us get back to rusting. This is one, change that affects iron articles and, slowly destroys them. Since iron is used, in making bridges, ships, cars, truck, bodies and many other articles, the, monetary loss due to rusting is huge., The process of rusting can be, represented by the following equation:, Iron (Fe) + Oxygen (O2, from the air), + water (H2O) → rust (iron oxide Fe2O3), For rusting, the presence of both, oxygen and water (or water vapour), is essential., In fact, if the content of moisture in, air is high, which means if it is more, humid, rusting becomes faster., So, how do we prevent rusting?, Prevent iron articles from coming in, contact with oxygen, or water, or both., One simple way is to apply a coat of, paint or grease. In fact, these coats, should be applied regularly to prevent, rusting. Another way is to deposit a layer, of a metal like chromium or zinc on iron., , PHYSICAL AND CHEMICAL CHANGES, , 63, , 2020-21

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Oh, that is why my friend, Rita is always complaining, about iron articles rusting so, fast. She lives near the coast., , This process of depositing a layer of zinc, on iron is called galvanisation. The iron, pipes we use in our homes to carry water, are galvanised to prevent rusting., You know that ships are made of iron, and a part of them remains under water., On the part above water also, water, drops keep clinging to the ship’s outer, surface. Moreover, the water of the sea, contains many salts. The salt water, makes the process of rust formation, faster. Therefore, ships suffer a lot of, damage from rusting in spite of being, Stainless steel is made by mixing iron, with carbon and metals like, chromium, nickel and manganese. It, does not rust., , formed from their solutions. The process, is called crystallisation. It is an, example of a physical change., , Activity 6.9, (To be performed in the presence of, the teacher), CAUTION, Use only dilute sulphuric acid. Be, careful while boiling water., Take a cupful of water in a beaker, and add a few drops of dilute sulphuric, acid. Heat the water. When it starts, boiling add copper sulphate powder, slowly while stirring continuously, (Fig. 6.6). Continue adding copper, sulphate powder till no more powder can, be dissolved. Filter the solution. Allow, it to cool. Do not disturb the solution, when it is cooling. Look at the solution, after some time. Can you see the crystals, of copper sulphate? If not, wait for some, more time., , painted. So much so, that a fraction of, ship’s iron has to be replaced every year., Can you imagine the monetary loss to, the world?, , Copper sulphate, , 6.4 CRYSTALLISATION, In Class VI you have learnt that salt can, be obtained by the evaporation of sea, water. The salt obtained in this manner, is not pure and the shape of its crystals, cannot be seen clearly. However, large, crystals of pure substances can be, , Crystals, Fig. 6.6 Crystals of copper sulphate, , SCIENCE, , 64, , 2020-21

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You have learnt about physical and, chemical changes. Try to identify, , changes that you observe around you, as physical or chemical changes., , Keywords, Chemical change, Chemical reaction, , Crystallisation, Galvanisation, , Physical change, Rusting, , What you have learnt, n, , Changes can be of two types, physical and chemical., , n, , Physical changes are changes in the physical properties of substances., No new substances are formed in these changes. These changes may, be reversible., , n, , In chemical changes new substances are produced., , n, , Some substances can be obtained in pure state from their solutions by, crystallisation., , Exercises, 1. Classify the changes involved in the following processes as physical or, chemical changes:, (a), , Photosynthesis, , (b), , Dissolving sugar in water, , (c), , Burning of coal, , (d), , Melting of wax, , (e), , Beating aluminium to make aluminium foil, , (f ), , Digestion of food, , 2. State whether the following statements are true or false. In case a, statement is false, write the corrected statement in your notebook., (a), , Cutting a log of wood into pieces is a chemical change. (True/False), , (b), , Formation of manure from leaves is a physical change. (True/False), , PHYSICAL AND CHEMICAL CHANGES, , 65, , 2020-21

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(c), , Iron pipes coated with zinc do not get rusted easily. (True/False), , (d), , Iron and rust are the same substances. (True/False), , (e), , Condensation of steam is not a chemical change. (True/False), , 3. Fill in the blanks in the following statements:, (a), , When carbon dioxide is passed through lime water, it turns milky, due to the formation of _________., , (b), , The chemical name of baking soda is _________., , (c), , Two methods by which rusting of iron can be prevented are, _________ and _________., , (d), , Changes in which only _________ properties of a substance change, are called physical changes., , (e), , Changes in which new substances are formed are called _________, changes., , 4. When baking soda is mixed with lemon juice, bubbles are formed with, the evolution of a gas. What type of change is it? Explain., 5. When a candle burns, both physical and chemical changes take place., Identify these changes. Give another example of a familiar process in, which both the chemical and physical changes take place., 6. How would you show that setting of curd is a chemical change?, 7. Explain why burning of wood and cutting it into small pieces are, considered as two different types of changes., 8. Describe how crystals of copper sulphate are prepared., 9. Explain how painting of an iron gate prevents it from rusting., 10. Explain why rusting of iron objects is faster in coastal areas than in, deserts., 11. The gas we use in the kitchen is called liquified petroleum gas (LPG). In, the cylinder it exist as a liquid. When it comes out from the cylinder it, becomes a gas (Change – A) then it burns (Change – B). The following, statements pertain to these changes. Choose the correct one., (i), , Process – A is a chemical change., , (ii), , Process – B is a chemical change., , (iii), , Both processes A and B are chemical changes., , (iv), , None of these processes is a chemical change., , 12. Anaerobic bacteria digest animal waste and produce biogas (Change – A)., The biogas is then burnt as fuel (Change – B). The following, statements pertain to these changes. Choose the correct one., (i), , Process – A is a chemical change., , (ii), , Process – B is a chemical change., SCIENCE, , 66, , 2020-21

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(iii), , Both processes A and B are chemical changes., , (iv), , None of these processes is a chemical change., , Extended Learning — Activities and Projects, 1. Describe two changes that are harmful. Explain why you consider them, harmful. How can you prevent them?, 2. Take three glass bottles with wide mouths. Label them A, B and C. Fill, about half of bottle A with ordinary tap water. Fill bottle B with water, which has been boiled for several minutes, to the same level as in A. In, bottle C, take the same boiled water and of the same amount as in other, bottles. In each bottle put a few similar iron nails so that they are completely under water. Add a teaspoonful of cooking oil to the water in, bottle C so that it forms a film on its surface. Put the bottles away for a, few days. Take out nails from each bottle and observe them. Explain, your observations., 3. Prepare crystals of alum., 4. Collect information about the types of fuels used for cooking in your, area. Discuss with your teachers/parents/others which fuels are less, polluting and why., , Did you know?, Near the Qutub Minar in Delhi, stands an iron pillar (Fig. 6.7), which is more than 7 metres, high. It weighs more than 6000 kg., It was built more than 1600 years, ago. After such a long period it has, not rusted. For its quality of rust, resistance it has been examined, by scientists from all parts of the, world. It tells something about the, advances India had made in, metal technology as back as, 1600 years ago., , Fig. 6.7 Iron pillar, , PHYSICAL AND CHEMICAL CHANGES, , 67, , 2020-21