Water of crystallization or the water of hydration is the water that occurs in the crystals. It is denoted as hydrated compound $$nH_2O $$. For example, gypsum is a soft mass having the formula $$ CaSO_4.2H_2O$$. On heating gypsum, plaster of paris is formed by losing the water of crystallization. Plaster of paris is a hard mass with the formula $$ CaSO_4.dfrac{1}{2}H_2O$$.