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.
| Remember |
| 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 |
| 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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