Copper(II) sulfate

Copper sulfate is probably the most recognisable coloured salt in inorganic chemistry: its pentahydrate CuSO₄·5H₂O forms translucent cobalt-blue crystals whose colour comes from allowed d-d transitions of the Cu²⁺ ion in the octahedral symmetry of aquo-complexes. The anhydrous form, white, only reproduces this colour upon contact with water — a property exploited to detect water traces in organic solvents (purity test).

Historically, its fame rests on the "Bordeaux mixture" devised in 1885 by Pierre-Marie Alexis Millardet (University of Bordeaux) after the invasion of Phytophthora infestans downy mildew was ravaging French vineyards. The classical formulation mixes ~1 % CuSO₄ and ~1 % Ca(OH)₂ in water, forming a copper hydroxide precipitate that sticks to leaves. This innovation saved the European vineyard and is still used 140 years later, including in organic farming (Cu²⁺ as a mineral fungicide) — although copper accumulation in soils is now an ecotoxicological concern.

Industrially, about 200 kt of CuSO₄ is produced annually, mainly by acid leaching of oxidised copper ores or as a co-product of electrolytic refining. Its applications cover swimming-pool algaecide, pre-treatment agent before electroplating, Fehling reagent for glucose assay in clinical biochemistry (in competition with the modern Fehling test), and elementary chemistry teaching — its visible crystallisation and the hydroxide precipitation by NaOH are among the great classics of high-school chemistry labs.