Which of the following are chemical changes?

(i) Decaying of wood

(ii) Burning of wood

(iii) Sawing of wood

(iv) Hammering of a nail into a piece of wood

(i) Decaying of wood and (ii) Burning of wood are chemical changes because in these processes, the chemical composition of wood is changed and new substances are formed. These changes are irreversible which cannot be converted back into their original form because chemical reactions are irreversible in nature.
In (iii) Sawing of wood and (iv) Hammering of a nail into a piece of wood, chemical composition and properties of wood are not changed, therefore, these are physical changes.