Brian Bennett 12/20/98
Cu(s) + 2 H2SO4(l) + 3 H2O(l) 
CuSO4·5 H2O(s) + SO2(g)
The beautiful blue color arises from water molecules attached directly to the copper(II) ion. The water/copper ion complex absorbs photons of yellow or red light. Absorption of a photon promotes an electron from the water to the copper(II) ion. Since only yellow or red light is absorbed, blue light is transmitted, and the crystals appear blue.
If blue crystalline CuSO4·5 H2O is heated strongly, the hydration water is driven off, leaving white powdery CuSO4 ("anhydrous" copper(II) sulfate). The blue will reappear if the anhydrous copper sulfate is exposed to even small amounts of moisture, making it sometimes useful as a spot test for water.
Four waters are directly bound to the copper to form a Cu(H2O)42+ ion, with the copper at the center of a square formed by the oxygens of the waters. The crystal structure of copper(II) sulfate pentahydrate can be pictured as an array of these Cu(H2O)42+ ions bridged by sulfate ions (SO42-) and water molecules, with one sulfate and one water per Cu(H2O)4+2 ion.
Copper(II) sulfate is used in:
- controlling the growth of algae in waterways, reservoirs, and swimming pools;
- controlling molds and funguses (as a component of "Bordeaux mixture");
- providing dietary copper in animal feeds;
- copper plating (depositing copper on metal) and electroforming (depositing copper on nonmetals);
- dying and calico printing, as a mordant (fixative);
- making Daniell cells, a type of battery popular in the 19th century;
- synthesis of other compounds of copper;
- crystallization of certain enzymes;
- fertilizers, as a source of copper;
- blue-green flame and firework colors;
Copper sulfate's toxicity has curtailed its use as an algaecide and fungicide. (In the 19th century it was actually used as a blue food coloring!)