A solution of copper(II) sulfate is blue.
The color comes mainly from hydrated copper(II) ions in water, often represented as the hexaaquacopper(II) ion, [Cu(H₂O)₆]²⁺. These ions absorb some wavelengths of visible light and transmit or reflect blue light. The sulfate ion itself is colorless in solution.
Copper(II) sulfate is commonly encountered as the pentahydrate, CuSO₄·5H₂O, which forms bright blue crystals and is traditionally called blue vitriol or bluestone. When heated, it loses its water of crystallization and becomes pale or gray-white anhydrous copper(II) sulfate. Adding water restores the hydrated blue form and releases heat.
The shade can change with concentration and with other ligands. Adding concentrated hydrochloric acid can shift the color toward green or yellow-green as chloride complexes form, while excess ammonia produces a much deeper royal-blue copper–ammonia complex. Copper(II) sulfate is used in electroplating, crystal-growing demonstrations, fungicides and laboratory tests, though copper salts are toxic and should be handled carefully.