The blue colour of copper sulphate crystals ($CuSO_4\cdot 5H_2O$) is due to

1. $CuSO_4\cdot 5H_2O$ contains 5 water molecules bound in the crystal — its **water of crystallisation**. 2. This water gives the crystals their blue colour; on heating, the water leaves and the solid turns white. 3. Adding water restores the blue colour. _Source: Karnataka SSLC (KSEEB) Class 10 Science, Ch2 'Acids, Bases and Salts', §2.4_