1. A piece of iron is heated until it becomes red-hot. It then becomes soft and is beaten to the
desired shape. What kinds of changes are observed in this process-reversible or irreversible?
2. Riya had bought a new bottle of pickle from the market. She tried to open the metal cap to taste
it but could not do so. She then took a bowl of hot water and immersed the upper end of the
bottle in it for five minutes. She could easily open the bottle now. Can you give the reason for
this?
3. Give one example in each case:
(a) Change which occurs on heating but can be reversed.
(b) Change which occurs on heating but cannot be reversed.
(c) Change which occurs on cooling but can be reversed.
(d) Change which occurs on mixing two substances, but can be reversed.
(e) Change which occurs on mixing two substances, but cannot be reversed.
4. A potter working on his wheel-shaped a lump of clay into a pot. He then baked the pot in an
oven. Do these two acts lead to the same kind of changes or different? Give your opinion and
justify your answer.
Answers
Answer:
1 ) A piece of iron on heating can be beaten into desired shape. It is a reversible change because the object of desired shape can be again heated and beated to give the original shape.
2) Almost all metals expand when exposed to heat. So the metal cap of the jar will also expand in hot water and will be easily opened.
3) (a) Heating of an iron rod.
(b) Baking of chapati.
(c) Formation of ice from water.
(d) Formation of salt solution
(e) Mixing of cement and water
4) The shaping of clay into a pot is a physical change, as only the shape of the clay is changed This change can be reversed. Baking of pot in the oven brings about a change in the chemical properties of the clay, and therefore this change cannot be reversed.
Answer:
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