When iron reacts with copper sulphate, the iron molecule replaces the copper sulphate molecule, resulting in the formation of copper and ferrous sulphate. The balanced chemical equation for this reaction is:

Fe(s) + CuSO4(aq) → Cu(s) + FeSO4(aq)

Copper separates out of the solution while ferrous sulphate remains in solution. The brown material that forms on the iron is not rust, but rather copper that is being plated on. When a silvery iron bar is put in a copper sulfate solution, the solution slowly starts to change color – the bright blue color gradually turns green (the salt FeSO4 which forms in replacement has a green color). Metallic copper of a reddish color also starts to form.