By W.W.J. NICOL, M.A., D.Sc.
When a hydrated salt is dissolved, does it retain its water of crystallization, or does this latter cease to be distinguishable from the solvent water? Both views have found advocates among chemists who have looked at the question of solution, and both have been supported by arguments more or less to the point. But among the possible means of solving this question there is one which has entirely escaped the notice of those interested in the subject. And those who hold that water of crystallization exists in solution have been entirely oblivious of the fact that, while they are ready to accept the results of the modern science of thermo-chemistry, and to employ them to support their views on hydration, yet these very results, if correct, prove without a shadow of a doubt that water of crystallization does not exist in solution.
The proof is so clear and self-evident when once one's attention is directed to it, that, though I intend to develop it more fully on another occasion, I feel that it is better to publish an outline of it at once.
Thomsen has found that the heat of neutralization of the soluble bases of the alkalies and alkaline earths with sulphuric acid has a mean value of 31.150 c. within very narrow limits. When hydrochloric or nitric acid is employed, the value is 27.640 c., also within very narrow limits. Now, this agreement of the six bases in their behavior with sulphuric acid, much more of the seven bases with both HNO and HCl, is so close that it cannot be regarded as accidental, but, in the words of Meyer, the heat of formation of a salt in aqueous solution is a quantity made up of two parts, one a constant for the base, the other for the acid. But of the twenty salts thus formed, some are anhydrous in the solid state, others have water of crystallization, up to ten molecules in the case of NaSO. If water of crystallization exists in solution, it will be necessary to suppose that this agreement is accidental, which is absurd, as a glance at the probabilities will show. Thomsen himself expressly states that he regards the dissolved state as one in which the conditions are comparable for all substances; this would be impossible if water of crystallization were present.
A still stronger proof is afforded by the "avidity" of Thomsen or the "affinity" of Ostwald; both have worked on the subject, taking no account of water of crystallization, and the results, e.g., for HSO and HCl with NaHO, where water of crystallization may come in, are entirely confirmed by Ostwald's results on inversion and etherification, where there can be no water of crystallization.
The proof is complete, water of crystallization cannot be attached to the salt in solution, or if it is, no heat is evolved on union more than with solvent water. The alternative is to suppose that the whole of the above thermo-chemical results are coincidences.