What is gypsum? What happens when gypsum is heated to 100°C (373 K)?
Gypsum is a salt, which possesses two water molecules as water of cyrstallisation. It has the formula CaSO4.2H2O. On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate (CaSO4 . 1/2H2O). This is called Plaster of Paris.