A brown-yellow powder, CuCl2; r.d. 3.386; m.p. 620°C. It exists as a blue-green dihydrate (rhombic; r.d. 2.54; loses H2O at 100°C). The anhydrous solid is obtained by passing chlorine over heated copper. It is predominantly covalent and adopts a layer structure in which each copper atom is surrounded by four chlorine atoms at a distance of 0.23 and two more at a distance of 0.295. A concentrated aqueous solution is dark brown in colour due to the presence of complex ions such as [CuCl4]2−. On dilution the colour changes to green and then blue because of successive replacement of chloride ions by water molecules, the final colour being that of the [Cu(H2O)6]2+ ion. The dihydrate can be obtained by crystallizing the solution.