Which of the following are chemical changes?
$(a)$ Growth of a plant
$(b)$ Rusting of iron
$(c)$ Mixing of iron filings and sand
$(d)$ Cooking of food
$(e)$ Digestion of food
$(f)$ Freezing of water
$(g)$ Burning of a candle

(A, B, D, E, G) Chemical changes involve a change in the chemical composition of a substance,resulting in the formation of new substances.
$(a)$ Growth of a plant: Involves complex biochemical reactions.
$(b)$ Rusting of iron: Iron reacts with oxygen and moisture to form iron oxide.
$(d)$ Cooking of food: Involves chemical breakdown of nutrients.
$(e)$ Digestion of food: Enzymes break down complex molecules into simpler ones.
$(g)$ Burning of a candle: Combustion is a chemical reaction.
Mixing of iron filings and sand $(c)$ and freezing of water $(f)$ are physical changes because no new chemical substances are formed.