Q. How many molecules of water Gypsum has, as a water of crystallisation? Answer:
2
Notes: Gypsum has two molecules of water as water of crystallisation. So, the chemical formula for hydrated gypsum becomes CaSO4.2H2O. This gypsum on getting heated loses water molecules and becomes calcium sulphate hemihydrate (CaSO4.1/2H2O). This is known as plaster of Paris.