Copper(II) sulfate exists in several hydration states. The most common form is copper(II) sulfate pentahydrate (CuSO₄·5H₂O), which is a bright blue crystalline solid. The blue colour is due to the presence of hydrated copper(II) ions, [Cu(H₂O)₄]²⁺, where water molecules act as ligands coordinated to the Cu²⁺ ion. The d-d electronic transitions within the Cu²⁺ ion, influenced by the surrounding water ligands, cause the absorption of certain wavelengths of visible light (specifically red-orange light), resulting in the complementary blue colour being observed. When copper(II) sulfate pentahydrate is heated, it loses its water of crystallization. The blue pentahydrate first turns into pale blue CuSO₄·3H₂O, then blue-green CuSO₄·H₂O, and finally becomes anhydrous copper(II) sulfate (CuSO₄), which is a white powder. In the anhydrous state, the crystal structure changes, and the Cu²⁺ ion is no longer surrounded by water ligands in the same way. This alters the energy levels of the d orbitals, and the absorption characteristics change such that no visible light is strongly absorbed, causing the compound to appear white.