water molecules forming an essential part of the crystal structure of some compounds.

water of crystallisation means the number of water molecules present in one formula unit of a salt. means, for eg:

CaSO4.2H2O this iz called Calcium Sulpahte di-hydrate or is popularly known as gypsum.

u notice CaSO4(Calcium Sulphate) is a one formula unit of a salt and is crystalised with 2 water molecules (2H2O)...

similarly CaSO4.1/2H20 Calcium Sulphate hemi-hydrate or popularly known as Plaster of Paris.

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Some salts,while crystallising out from their solutions ,unite with a definite quantity of water ,which is known as water of crystallisation.