Copper sulphate pentahydrate crystal structure

Note that the formula of solid copper sulphate pentahydrate for example should be written precisely as [Cu(H20)4] [S04(H20)]. The usual formula, CuS04.5H20 does not account for the fact that there are two different types of water molecules (copper-water and sulphate-water) in the crystal structure. This can easily be proved. On heating, first four molecules of water are released from crystalline copper sulphate, at around 120°C, while the fifth molecule can only be removed at a much higher temperature, 240°C. 

Heat is liberated when adding water to anhydrous copper sulphate because a new crystal lattice forms in response to strong, new bonds forming between the water and Cu2+ and SO2- ions. As corroborative evidence of a change in the crystal structure, note how anhydrous copper sulphate is off-white but the pentahydrate is blue. 

Preparation of a Complex Ammonium Salt of Copper(II). Dissolve 0.5 g of finely triturated copper(II) sulphate pentahydrate in 12.5 ml of a 15% ammonia solution. If the solution is turbid, filter it. Slowly add 7.5 ml of ethanol to the filtrate and let it stand for a few hours in the cold. Filter off the formed crystals, wash them first with a mixture of ethanol and a concentrated ammonia solution (1 1), and then with ethanol and ether. Dry them at room temperature. Into what ions does the product dissociate in the solution Consider the structure of the complex ion from the viewpoint of the valence bond theory. 

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