Water of crystallization

When crystals of certain salts are formed, they do so with a definite number of molecules of water, chemically combined in a definite proportion. Water of crystallization is the number of water molecules, chemically combined in a definite molecular proportion, with the salt in its crystalline state. This water is responsible for the geometric shape and colour of the crystals.

A substance containing water of crystallization is called a hydrous substance or a hydrate. This water can be expelled, by heating, and then the salt is said to have become anhydrous.

Some Hydrous Substances

Name Chemical Formula Common Name
Barium chloride BaCl2.2H2O
Calcium chloride CaCl2.6H2O Dow flake
Calcium nitrate Ca(NO3)2.4H2O Lime salt petre or Norwegian salt petre
Calcium sulphate CaSO4.2H2O Gypsum
Calcium sulphate
semi hydrate
(CaSO4) .½. H2O Plaster of Paris
Copper (II) chloride CuCl2.2H2O
Copper (II) sulphate CuSO4.5H2O Blue vitriol
Ferrous sulphate FeSO4.7H2O Green vitriol
Zinc sulphate ZnSO4.7H2O White vitriol
Magnesium sulphate MgSO4.7H2O Epsom salt
Potassium aluminium
sulp hate
K2SO4.Al2 (SO4) 3.24H2O Potash alum
Sodium carbonate
deca hydrate
Na2CO3,10H2O Washing soda (crystal)
Sodium sulphate Na2SO4.10H2O Glauber’s salt

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