Question

What is meant by “water of crystallisation” in a substance? Explain with an example.

Answer 1

The water molecules which form part of the structure of a crystal (of a salt) are called water of crystallisation. Copper sulphate crystals contain 5 molecules of water of crystallisation in one formula unit and hence are written as CuSO₄.5H₂O. It is called copper sulphate pentahydrate ('Pentahydrate' means 'five molecules of water').