When a substance dissolves in water, its molecules/ions are surrounded by water molecules. In some cases, it is possible that water molecules coordinate with the solute ions, i.e. water molecules become part of a chemical structure with the solute called a coordination complex. Not only water can coordinate with ions but also other substances form coordination complexes.
The electronic structure of the transition elements allows the formation of coordination complexes in which the metal ions bond with other ions or molecules to form a structure with a characteristic 3-dimensional geometric shape. The shape adopted by the complex may be associated with a characteristic colour. Copper forms easily coordination complexes with water and other substances. Anhydrous copper sulphate has a dirty white colour, whereas the hydrate CuSO4•5H2O has a blue colour because copper is coordinated with water in the crystals. Dissolving either of these salts in water gives a blue solution because the copper ions coordinate with water molecules. Adding a small quantity of ammonia raises the pH of the solutions shift towards alkaline values and some copper hydroxide precipitates.
Cu2++ 2OH– → Cu(OH)2
Copper hydroxide has a pale blue colour. If ammonia is added in excess the precipitate dissolves because a tetraammine complex of copper is formed with the ammonia and a royal blue solution is obtained.
Cu(OH)2+ 4NH3 → [Cu(NH3)4]2++2OH–
Adding some sodium chloride to the starting copper sulphate solution causes a bright green due to the formation of copper (II) chloride complexes.