1. The following changes cannot be reversed:
(d) Baking of a cake is irreversible.
(f) Making biogas from cow dung is irreversible.
(g) Burning of a candle is irreversible.
2. (d): expands first, then on cooling contracts and fits onto the wooden wheel.
Iron expands on heating. So the smaller iron rim expands on heating fits into the wooden wheel and on cooling it contracts to fit into the wooden wheel tightly. Hence it does not come out.
3. Here are examples for the above statements:
(a) Heating of an iron rod
(b) Baking of chapati
(f) Formation of ice from water
(d) Formation of salt solution
(e) Mixing of cement with water
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