Water Of crystallisation :
The water molecules which form part of the structure of a crystal are called water of crystallisation. The salts which contain water of crystallisation are called hydrated salts.
1)Copper Sulphate crystals contain 5 molecules of water of crystallisation( CuSO4.5H2O). It is blue in colour.
2)Sodium carbonate crystals contains 10 molecules of water of
When hydrated salts are heated strongly, they lose their water of crystallisation. By losing the water of crystallisation, the hydrated salts lose their regular shape and colour, and become colourless powdery substances,
The salts which have lost their water of crystallisation are called anhydrous salts.
For example, gypsum is a soft mass having the formula CaSO4.2H2O. On heating gypsum, plaster of Paris is formed by losing the water of crystallization. Plaster of Paris is a hard mass with the formula CaSO4.12H2O.
To show the water of crystallization of a substance: Copper sulphate crystals are blue in colour. When copper sulphate crystals are heated strongly, they lose water of crystallisation and form anhydrous copper sulphate which is white in colour.
Put 2g of copper sulphate crystals on a china dish and heat it using a Bunsen burner.
Observations: The blue colour crystals of Copper sulphate changes their colour to white powder removing 5 molecules of water of crystallization.
Conclusion: Crystalline substances contain water of crystallisation that are lost on heating.