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Physical and Chemical Changes 59 Physical and 6 Chemical Changes very day you come across many 6.1 PHYSICAL CHANGES changes in your surroundings. Activity 6.1 EThese changes may involve one or more substances. For example, your Cut a piece of paper in four square mother may ask you to dissolve sugar pieces. Cut each square piece further in water to make a cold drink. Making a into four square pieces. Lay these pieces sugar solution is a change. Similarly, on the floor or a table so that the pieces setting curd from milk is a change. acquire the shape of the original piece Sometimes milk becomes sour. Souring of paper (Fig. 6.1). of milk is a change. Stretched rubber Obviously, you cannot join the pieces band also represents a change. back to make the original piece, but is Make a list of ten changes you have there a change in the property of the noticed around you. paper? In this chapter we shall perform some Activity 6.2 activities and study the nature of these changes. Broadly, these changes are of Collect the chalk dust lying on the floor two kinds, physical and chemical. 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 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 Fig. 6.1 Paper pieces again? 58 SCIENCE 2021–22 Activity 6.4 6.3 and 6.4, water changed its state (from solid to liquid, or from gas to liquid). In Boil some water in a container. Do you Activity 6.5, the hack-saw blade see the steam rising from the surface of changed colour on heating. water? Hold an inverted pan by its Properties such as shape, size, colour handle over the steam at some distance and state of a substance are called its from the boiling water. Observe the physical properties. A change in which inner surface of the pan. a substance undergoes a change in its Do you see any droplet of water physical properties is called a physical there? change. A physical change is generally Activity 6.5 reversible. In such a change no new CAUTION substance is formed. Let us now consider the other kind Be careful while handling a flame. of change. Hold a used hack-saw blade with a pair of tongs. Keep the tip of the free 6.2 CHEMICAL CHANGE end of the blade on the gas stove. Wait A change with which you are quite for a few minutes. familiar is the rusting of iron. If you Does the colour of the tip of the blade leave a piece of iron in the open for some change? time, it acquires a film of brownish Remove the blade from the flame. substance. This substance is called rust Observe the tip once again after some and the process is called rusting time. (Fig. 6.2). Iron gates of parks or Does it get back its original colour? farmlands, iron benches kept in lawns In Activities 6.1 and 6.2 above, you and gardens, almost every article of iron, saw that paper and a piece of chalk kept in the open gets rusted. At home underwent changes in size. In Activities you must have seen shovels and spades getting rusted when exposed to the Fig. 6.2 Rusting iron PHYSICAL AND CHEMICAL CHANGES 59 2021–22 atmosphere for some time. In the light (Fig. 6.3). When it is completely kitchen, a wet iron pan (tawa) often gets burnt it leaves behind a powdery ash. rusted if left in that state for some time. Does the ash look like the Rust is not iron. It is different from iron magnesium ribbon? on which it gets deposited. The change can be represented by Let us consider a few more changes the following equation: → where new substances are formed. Magnesium (Mg) + Oxygen (O2) Activity 6.6 Magnesium oxide (MgO) (To be demonstrated by the teacher) The equations here are different from CAUTION those in mathematics. In equations It is dangerous to look for long at the of this kind, the arrow implies burning magnesium ribbon. The ‘becomes’. No attempt should be made teachers should advise children not to balance chemical equations at this to stare at the burning ribbon. stage. Get a small piece of a thin strip or Collect the ash and mix it with a ribbon of magnesium. Clean its tip with small amount of water. Stir the mixture sandpaper. Bring the tip near a candle (aqueous solution) well. Test the mixture flame. It burns with a brilliant white 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. Fig. 6.3 Magnesium ribbon burning Magnesium hydroxide is another new 60 SCIENCE 2021–22 substance formed by mixing magnesium colour of the solution from blue to oxide with water. green is due to the formation of iron Activity 6.7 sulphate, a new substance. The brown deposit on the iron nail is copper, (To be demonstrated by the teacher) another new substance. We can write Dissolve about a teaspoonful of copper the reaction as: sulphate (blue vitriol or neela thotha) in Copper sulphate solution (blue) + Iron about half a cup of water in a glass → Iron sulphate solution (green) tumbler or a beaker. Add a few drops of + Copper (brown deposit) dilute sulphuric acid to the solution. Activity 6.8 You should get a blue coloured solution. Save a small sample of the solution in a Take about a teaspoonful of vinegar in test tube or a small glass bottle. Drop a a test tube. Add a pinch of baking soda nail or a used shaving blade into the to it. You would hear a hissing sound remaining solution. Wait for half an and see bubbles of a gas coming out. hour or so. Observe the colour of the Pass this gas through freshly prepared solution. Compare it with the colour of lime water as shown in Fig. 6.5. the sample solution saved separately What happens to the lime water? (Fig. 6.4). 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 The change in the test tube is as of the solution? follows: Take out the nail or the blade. Vinegar (Acetic acid) + Baking soda Has it changed in any way? (Sodium hydrogencarbonate) → The changes that you notice are Carbon dioxide + other substances due to a reaction between copper The reaction between carbon dioxide sulphate and iron. The change of and lime water is as follows: PHYSICAL AND CHEMICAL CHANGES 61 2021–22 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. Carbon dioxide Chemical changes are very important in our lives. All new substances are formed as a result of chemical changes. Vinegar + For example, digestion of food in our Baking soda body, ripening of fruits, fermentation of Lime grapes, etc., happen due to series of water chemical changes. A medicine is the end product of a chain of chemical reactions. Useful new materials, such as plastics Fig. 6.5 Set up to pass gas through lime water and detergents, are produced by chemical reactions. Indeed, every new Carbon dioxide (CO ) + Lime water 2 material is discovered by studying [Ca(OH) ] → Calcium Carbonate 2 chemical changes. (CaCO ) + Water (H O) 3 2 We have seen that one or more new When carbon dioxide is passed substances are produced in a chemical through lime water, calcium carbonate change. In addition to new products, is formed, which makes lime water milky. the following may accompany a chemical The turning of lime water into milky is a change: standard test of carbon dioxide. You will n Heat, light or any other radiation use it in Chapter 10 to show that the air (ultraviolet, for example) may be given we breathe out is rich in carbon dioxide. off or absorbed. In Activities 6.6–6.8, you saw that n Sound may be produced. in each change one or more new substances were formed. In Activity 6.6, n A change in smell may take place or the ash was the new substance formed a new smell may be given off. when magnesium was burnt in air. In n A colour change may take place . Activity 6.7, the reaction of copper n A gas may be formed. sulphate with iron produced iron Let us look at some examples. sulphate and copper. Both of these are You saw that burning of magnesium new substances.
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