Copper sulphate crystals seem to be dry but they contain water of crystallization. When we heat the crystals, water is removed and the salt turns white. If we moisten the crystals again with water, then blue colour of crystals reappears.
Water of crystallization is the fixed number of water molecules present in one formula unit of a salt. Chemical formula for hydrated copper sulphate is CuSO4.5H2O. There is one other salt which possesses water of crystallization is gypsum. Its chemical formula is CaSO4.2H2O.