Changes Around Us

Changes happen around us all the time. Seasons change, buds open into flowers, ice melts, milk turns into curd and a balloon swells when we blow into it. In this chapter you will learn to group changes into those that can be reversed and those that cannot. Folding paper, stretching a rubber band, melting wax and freezing water are reversible changes, because the substance can be brought back to its original form. Burning paper, cooking food, ripening fruit and setting curd are irreversible changes, because the original substance cannot be got back. The chapter also shows that heating causes many changes. Some substances melt or evaporate on heating and return to their original state on cooling, while others, like wood or a candle wick, burn and change completely. Metals expand when heated and contract when cooled, which is why a blacksmith heats an iron rim before fitting it onto a wooden wheel. Mixing substances, such as plaster of Paris with water, can also cause changes that cannot be reversed.

Detailed Worksheet: Changes Around Us Section A - Definitions (10 marks) 1. What is a reversible change? Give two examples. (2 marks) 2. What is an irreversible change? Give two examples. (2 marks) 3. Define expansion and contraction with one example of each. (2 marks) 4. What is melting and what is freezing? How are these two changes related? (2 marks) 5. What is evaporation and what is condensation? Give one daily-life example of each. (2 marks) Section B - Calculations and Applications (15 marks) 6. Classify the following as reversible or irreversible: (i) blowing up a balloon (ii) burning a matchstick (iii) melting butter (iv) cooking rice (v) dissolving sugar in water (vi) growth of a baby into an adult. (3 marks) 7. An iron rim has a diameter slightly smaller than the wooden wheel it must fit. (i) What does the blacksmith do to the rim before fitting it? (ii) Why is cold water poured on the rim afterwards? (iii) Which property of metals is being used? (3 marks) 8. A student heats 50 g of ice in a beaker until it all melts, then continues heating until some water evaporates, and finally cools the remaining water in a freezer. (i) Name the change at each step. (ii) Which steps are reversible? (iii) Will the mass of the final ice be more or less than 50 g? Give a reason. (3 marks) 9. Give one application of each in daily life: (i) expansion of metals on heating (ii) contraction on cooling (iii) setting of plaster of Paris. (3 marks) 10. A candle is burnt for some time. (i) Which change in the candle is reversible? (ii) Which change is irreversible? (iii) Why is burning a candle a combination of both types of change? (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of the activity to show that a metal expands on heating, using a metal ball and ring (or an iron rim and wheel). Label the burner, ring and ball. State what happens when the ball is heated and when it is cooled. (4 marks) 12. Draw a flowchart showing the changes of state of water: ice -> water -> water vapour -> water -> ice. Name each change on the arrows. (3 marks) 13. Draw a labelled diagram of a candle burning and label the parts where wax melts, where it burns, and where it solidifies again. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Leela heats a piece of paper and a small amount of wax separately. (i) Describe what happens in each case. (ii) Which change can be reversed? (iii) Why can the burnt paper not be got back? (iv) Give one more example each of a change that can and cannot be reversed by cooling. (5 marks) 15. A doctor uses plaster of Paris to set a broken bone. (i) What happens when plaster of Paris is mixed with water? (ii) Is this change reversible? (iii) Why is it suitable for setting fractures? (iv) Name two other changes caused by mixing substances. (5 marks) 16. Electric wires hung between poles sag more in summer than in winter. (i) Explain this observation. (ii) What would happen if the wires were stretched tightly in summer? (iii) Why are small gaps left between rails on railway tracks? (iv) Name the property involved. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all diagrams neatly in pencil and label every part. Give a reason for each classification.