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Practice each question, then open the answer or worked solution.
Look at Fig. 6.1 which shows three situations
(a) A burning candle
(b) An extinguished candle
(c) Melting wax.
Which of these shows a reversible change and why?
Ans: The correct answer is option (c) the melting wax. Melted wax can be changed into its solid or viscous state on cooling. Therefore, it is a reversible change.
A piece of iron is heated till it becomes red-hot. It then becomes soft and is beaten to a desired shape. What kinds of changes are observed in this process– reversible or irreversible?
Ans: The temperature change in the iron piece is a reversible process. A piece of iron will expand when heated and can be changed into the desired shape. That desired thing/shape can then be heated and beaten again to restore its original shape.
Paheli 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?
Ans: Almost all metals expand when exposed to heat. As a result, when the jar is submerged in hot water, the metal cap expands and opens effortlessly.