Science Class 7 Ch 6 PDF

6 Physical and Chemical Changes E very day you come across many 6.1 PHYSICAL CHANGES changes in your surroundings. These changes may involve one Activity 6.1...

6 Physical and Chemical Changes E very day you come across many 6.1 PHYSICAL CHANGES changes in your surroundings. These changes may involve one Activity 6.1 ed 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 h sugar solution is a change. Similarly, on the floor or a table so that the pieces pu T setting curd from milk is a change. acquire the shape of the original piece is Sometimes milk becomes sour. Souring of paper (Fig. 6.1). re ER of milk is a change. Stretched rubber Obviously, you cannot join the pieces bl 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? be C In this chapter we shall perform some activities and study the nature of these Activity 6.2 o N changes. Broadly, these changes are of Collect the chalk dust lying on the floor two kinds, physical and chemical. near the blackboard 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 tt Take some ice in a glass or plastic tumbler. Melt a small portion of ice by placing the tumbler in the sun. You have no 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 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 ed there? change. A physical change is generally Activity 6.5 reversible. In such a change no new CAUTION substance is formed. h Let us now consider the other kind pu T Be careful while handling a flame. of change. is Hold a used hack-saw blade with a re ER pair of tongs. Keep the tip of the free 6.2 CHEMICAL CHANGE bl end on the flame of a gas stove. Wait for A change with which you are quite 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 be C change? time, it acquires a film of brownish Remove the blade from the flame. substance. This substance is called rust o N 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 tt no Fig. 6.2 Rusting iron PHYSICAL AND CHEMICAL CHANGES 59 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) → Magnesium oxide (MgO) Activity 6.6 ed (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 h of this kind, the arrow implies pu T burning magnesium ribbon. The ‘becomes’. No attempt should be made is teachers should advise children not to balance chemical equations at this re ER to stare at the burning ribbon. stage. bl 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 be C (aqueous solution) well. Test the mixture flame. It burns with a brilliant white with blue and red litmus papers. o N 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 tt equation: Magnesium oxide (MgO) + Water (H2O) → Magnesium hydroxide [Mg(OH)2] no 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 substance formed by mixing magnesium colour of the solution from blue to oxide with water. green is due to the formation of iron sulphate, a new substance. The brown Activity 6.7 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) ed dilute sulphuric acid to the solution. You should get a blue coloured solution. Activity 6.8 Save a small sample of the solution in a Take about a teaspoonful of vinegar in h test tube or a small glass bottle. Drop a a test tube. Add a pinch of baking soda pu T nail or a used shaving blade into the to it. You would hear a hissing sound is remaining solution. Wait for half an re ER and see bubbles of a gas coming out. hour or so. Observe the colour of the Pass this gas through freshly prepared bl 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). be C o N © Iron sulphate Copper sulphate Fig. 6.4 Change in colour of the copper sulphate solution due to reaction with iron tt Do you see any change in the colour The change in the test tube is as of the solution? no 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  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 + Baking soda For example, if a metal is to be extracted from an ore, such as iron from iron ore, ed Lime we need to carry out a series of chemical water changes. A medicine is the end product of a chain of chemical reactions. Useful h new materials, such as plastics and pu T Fig. 6.5 Set up to pass gas through lime water detergents, are produced by chemical is re ER reactions. Indeed, every new material Carbon dioxide (CO2) + Lime water is discovered by studying chemical bl [Ca(OH) 2 ] → Calcium Carbonate changes. (CaCO3) + Water (H2O) We have seen that one or more new When carbon dioxide is passed substances are produced in a chemical be C through lime water, calcium carbonate change. In addition to new products, is formed, which makes lime water milky. the following may accompany a chemical o N The turning of lime water into milky is a change: standard test of carbon dioxide. You will „ 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 „ Sound may be produced. in each change one or more new „ A change in smell may take place or substances were formed. In Activity 6.6, the ash was the new substance formed a new smell may be given off. when magnesium was burnt. In Activity 6.7, „ A colour change may take place. the reaction of copper sulphate with iron „ A gas may be formed. tt produced iron sulphate and copper. Let us look at some examples. Both of these are new substances. You saw that burning of magnesium no Copper was deposited on the shaving ribbon is a chemical change. Burning blade of iron. In Activity 6.8, vinegar and of coal, wood or leaves is also a chemical baking soda together produced carbon change. In fact, burning of any dioxide, which turned lime water milky. substance is a chemical change. Can you name the new substance Burning is always accompanied by formed in this reaction? production of heat. 62 SCIENCE  Explosion of a firework is a chemical change. You know that such an explosion produces heat, light, sound We learnt in Chapter 1 that and unpleasant gases that pollute the plants produce their food by atmosphere. That is why you are advised a process called photosynthesis. not to play with fireworks. Can we call photosynthesis When food gets spoiled, it produces a chemical change? a foul smell. Shall we call this change a chemical change? ed Paheli said that even digestion You must have noticed that a slice of is a chemical change. an apple acquires a brown colour if it is not consumed immediately. If you have h not seen this change in colour, cut a pu T fresh slice of apple and keep it away for is 6.3 RUSTING OF IRON some time. Repeat the same activity with re ER a slice of potato or brinjal. The change Let us get back to rusting. This is one bl of colour in these cases is due to the change that affects iron articles and formation of new substances. Are not slowly destroys them. Since iron is used these changes chemical changes? in making bridges, ships, cars, truck be C In Chapter 5, you neutralised an acid bodies and many other articles, the with a base. Is neutralisation a chemical monetary loss due to rusting is huge. o N change? The process of rusting can be represented by the following equation: Iron (Fe) + Oxygen (O2, from the air) © A protective shield + water (H2O) → rust (iron oxide Fe2O3) For rusting, the presence of both You must have heard of the ozone layer oxygen and water (or water vapour) in our atmosphere. It protects us from is essential. the harmful ultraviolet radiation which In fact, if the content of moisture in come from the sun. Ozone absorbs this air is high, which means if it is more radiation and breaks down to oxygen. humid, rusting becomes faster. tt Oxygen is different from ozone. Can So, how do we prevent rusting? we call the breaking down of ozone a Prevent iron articles from coming in chemical change? no contact with oxygen, or water, or both. If ultraviolet radiation were not One simple way is to apply a coat of absorbed by ozone, it would reach the paint or grease. In fact, these coats earth’s surface and cause harm to us should be applied regularly to prevent and other life forms. Ozone acts as a rusting. Another way is to deposit a layer natural shield against this radiation. of a metal like chromium or zinc on iron. PHYSICAL AND CHEMICAL CHANGES 63  solutions. The process is called crystallisation. It is an example of a Oh, that is why my friend physical change. Rita is always complaining about iron articles rusting so Activity 6.9 fast. She lives near the coast. (To be performed in the presence of the teacher) This process of depositing a layer of zinc CAUTION ed on iron is called galvanisation. The iron pipes we use in our homes to carry water Use only dilute sulphuric acid. Be are galvanised to prevent rusting. careful while boiling water. You know that ships are made of iron h and a part of them remains under water. Take a cupful of water in a beaker pu T is On the part above water also, water and add a few drops of dilute sulphuric re ER drops keep clinging to the ship’s outer acid. Heat the water. When it starts boiling add copper sulphate powder bl surface. Moreover, the water of the sea contains many salts. The salt water slowly while stirring continuously makes the process of rust formation (Fig. 6.6). Continue adding copper faster. Therefore, ships suffer a lot of sulphate powder till no more powder can be C damage from rusting in spite of being be dissolved. Filter the solution. Allow it to cool. Do not disturb the solution o N Stainless steel is made by mixing iron when it is cooling. Look at the solution with carbon and metals like after some time. Can you see the crystals chromium, nickel and manganese. It of copper sulphate? If not, wait for some © does not rust. more time. painted. So much so, that a fraction of ship’s iron has to be replaced every year. Copper sulphate Can you imagine the monetary loss to the world? 6.4 CRYSTALLISATION tt In Class VI you have learnt that salt can be obtained by the evaporation of sea no water. The salt obtained in this manner is not pure and its crystals are small. The shape of the crystals cannot be seen clearly. However, large crystals of pure Crystals substances can be formed from their Fig. 6.6 Crystals of copper sulphate 64 SCIENCE  You have learnt about physical and changes that you observe around you chemical changes. Try to identify as physical or chemical changes. Keywords Chemical change Crystallisation Physical change Chemical reaction Galvanisation Rusting ed What you have learnt h pu T „ Changes can be of two types, physical and chemical. is „ Physical changes are changes in the physical properties of substances. re ER No new substances are formed in these changes. These changes may be reversible. bl „ In chemical changes new substances are produced. „ Some substances can be obtained in pure state from their solutions by crystallisation. be C o N 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 tt (e) Beating aluminium to make aluminium foil (f ) Digestion of food no 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  (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 _________. ed (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. h pu T (e) Changes in which new substances are formed are called _________ is changes. re ER 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. bl 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. be C 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 o N 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. tt (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. no 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. 66 SCIENCE  (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 ed 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 com- pletely under water. Add a teaspoonful of cooking oil to the water in h bottle C so that it forms a film on its surface. Put the bottles away for a pu T few days. Take out nails from each bottle and observe them. Explain is your observations. re ER 3. Prepare crystals of alum. bl 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. be C o N 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 tt metal technology as back as 1600 years ago. no Fig. 6.7 Iron pillar PHYSICAL AND CHEMICAL CHANGES 67