CBSE Class 6 Answered

(a) Reversible changes are changes that can be reversed to form the original substance.

Irreversible changes are changes that cannot be reversed to form the original substance.

(b) (i) If we inflate a balloon, the size and shape of the balloon undergoes a change. However, the original size and shape of the balloon can be obtained back by allowing the air to escape from the balloon. This means that the change that occurs by inflating a balloon can be reversed. But, if the balloon bursts after being inflated, then its original size and shape cannot be obtained back. Thus, in this case, the change cannot be reversed.

(ii) If we fold a piece of paper, then the shape and size of the paper undergoes a change. In this case, the original shape and size of the original paper can be obtained back. Thus, this change can be reversed. However, if we cut the piece of paper, the change in the shape and size of the paper cannot be reversed.

(iii) If we heat solid wax, it melts to form a liquid called molten wax. On cooling, the molten wax becomes solid again i.e. melting of wax is a change that can be reversed hence it is a reversible change. But, if we light a candle, then the wax present in it starts burning to produce carbon dioxide gas, water vapour, soot, heat and light. These products obtained by burning of wax cannot be combined to get back the original candle hence burning of wax is a change that cannot be reversed i.e. it is an irreversible change.