$(a)$ Write the chemical formula of hydrated copper sulphate and anhydrous copper sulphate. Giving an activity,illustrate how these two are interconvertible.
$(b)$ Write the chemical names and formulae of Plaster of Paris and gypsum.

(N/A) The chemical formula of hydrated copper sulphate is $CuSO_4 \cdot 5H_2O$ (blue vitriol),and anhydrous copper sulphate is $CuSO_4$ (white).
Activity: Take some copper sulphate crystals in a dry boiling tube and heat them. The blue colour of the crystals disappears,and the substance becomes white due to the loss of water of crystallisation. If we add a few drops of water to this anhydrous copper sulphate,the blue colour is restored.
$(b)$ Plaster of Paris: Calcium sulphate hemihydrate,$CaSO_4 \cdot \frac{1}{2}H_2O$.
Gypsum: Calcium sulphate dihydrate,$CaSO_4 \cdot 2H_2O$.