Question

What is meant by water of crystallisation ? how many water molecules of water are present in hydrated copper sulphate write it's formula . what colour change do you absorve when it is heated?

Answer 1

Water of crystallisation is the number of water molecules that combine chemically in definite molecular proportion, with the concerned salt in the crystalline state. This water is responsible for the geometric shape and colour of the crystals.
For example: the CuSO4.5H2O
Copper sulphate crystals are blue in colour. However, when copper sulphate is heated in a china dish for some time, the blue colour disappears and it becomes white-gray in colour. This is the anhydrous copper sulphate. The reaction is written as follows
Thus the blue colour of copper sulphate crystals is due to water of crystallisation, which are lost when it is heated. Hence the blue colour of solution disappears. This proves that copper sulphate contains water of crystallisation.

Answer 2

Water of crystallization is the fixed number of water molecules present in one formula unit of salt. (i) Five molecules of water. (ii) Formula -CuSO4.5H2O (iii) When heated, its colour changes from blue to white