$(a)$ Illustrate with an example that physical and chemical changes can take place together.
$(b)$ Which of the following are chemical changes?
$(i)$ Mixing of iron filings and sand
$(ii)$ Growth of plant
$(iii)$ Rusting of iron
$(iv)$ Freezing of water

(II, III) Burning of a candle: When a candle burns,the wax melts,which is a physical change. Simultaneously,the wax reacts with oxygen to produce $CO_2$ and $H_2O$ vapor,which is a chemical change. Thus,both changes occur together.
$(b)$ The chemical changes are:
$(ii)$ Growth of plant: This involves complex metabolic processes and permanent changes in the organism's structure.
$(iii)$ Rusting of iron: This is a chemical reaction where iron reacts with oxygen and moisture to form hydrated iron$(III)$ oxide.