CBSE Class 10 Science 2020 · Paper 31/2/1 · Question 16(a)

(i) Plaster of Paris is calcium sulphate hemihydrate, ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$. Two formula units of ${\text{CaSO}}_4$ share one molecule of water of crystallisation, so per formula unit there is half a water molecule. (ii) Sodium hydrogen carbonate is alkaline (a mild base), so it neutralises the excess hydrochloric acid in the stomach and relieves acidity: ${\text{NaHCO}}_3+\text{HCl}⟶\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2$. (iii) Blue copper sulphate crystals contain water of crystallisation, ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$. Strong heating removes this water and leaves white anhydrous copper sulphate: ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}\stackrel{\text{heat}}{\to }{\text{CuSO}}_4+5{\text{H}}_2\text{O}$.