WBCS Preliminary Examination 2018 — General Studies · Question 109 of 200

Answer: (B) Gypsum

Plaster of Paris is calcium sulphate hemihydrate, $CaSO_4\cdot\tfrac{1}{2}H_2O$, made by heating gypsum, $CaSO_4\cdot 2H_2O$, to about 100-120°C (373-393 K) so that it loses three-fourths of its water of crystallisation. On adding water it sets again into hard gypsum, so it is used for plastering fractured bones, casts, statues and false ceilings. Heating gypsum too strongly gives 'dead burnt plaster', which does not set.