CBSE Class 10 Science 2020 · Paper 31/2/3 · Question 17(a)

(a) Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt, for example the five water molecules in each formula unit of blue copper sulphate crystals, ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$. (b) (i) When baking soda is heated, it decomposes into sodium carbonate, water and carbon dioxide; the carbon dioxide makes the cake or bread rise and become soft and spongy: $2{\text{NaHCO}}_3\stackrel{\text{heat}}{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$ (ii) When gypsum is heated at $373$ K, it loses one and a half of its two water molecules per formula unit and becomes Plaster of Paris (calcium sulphate hemihydrate): ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}\stackrel{373\text{K}}{\to }{\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+1\frac{1}{2}{\text{H}}_2\text{O}$