Hydrated copper sulfate (CuSO₄·5H₂O) is blue in color due to the presence of water molecules in its crystalline structure. The water molecules form a complex with copper ions, which gives the crystal its characteristic blue hue. When heated, the water evaporates, and the salt loses its blue color. This is because the removal of water alters the structure of the copper ions and the ligand field around them, causing the color to fade and the salt to turn into an anhydrous form, which is white. This process is reversible: if the anhydrous copper sulfate is exposed to water, it will absorb water and return to its blue hydrated form.