Anhydrous copper sulphate. Image 1 of 3. This copper salt (CuSO4) usually has a blue colour, but this is due to the presence of water of crystallisation within the molecular structure of the crystals. Hydrated copper sulphate has the formula CuSO4.5H2O. Heating removes the five water molecules (H2O), leading to the colour being lost. The process is reversible, and adding water to the anhydrous crystals restores their blue colour. For a sequence showing the addition of water to these crystals, see images A500/380-382.