Hydrates and electrolyte solutions

The extra field of the subgroup ions certainly plays a part in the low solubility of these compounds, but it cannot be said that this alone reduces solubility since AgF, in which the fluorine ion is particularly small, is freely soluble in contrast to AgCl and the other silver halides. It must be kept in mind that, in general, lattice energy 

Finally, it must be remembered that the hydration is also influenced by the structure of the ion. In view of the fact that the positive pole of a water molecule attaches itself to the negative ion, the molecule will be repelled by the positive nucleus of the positive ion. This means that the ion of a strong acid has a small hydration energy and will therefore have a smaller heat of solution and hence the tendency to form insoluble compounds. This effect is shown when CIOT combines with a large ion, and must be taken into account in explaining the solubility behaviour of caesium salts, which 

It is obvious that a non-polar molecule cannot penetrate into the crystals of an electrolyte, nor dissolve it. This explains why, for example, NaCl is insoluble in benzene, CC14, CS2, etc., which are non-polar. On the other hand, nitrobenzene is not a solvent for salts either, yet its dipole moment of 4 X 10-18 is larger than that of water. It must, however, be remembered that the molecule C6H6N02 is also much larger than that of water, and consequently the larger dipole cannot approach as closely to the charges of the ions. A liquid, to produce easy dissociation, must have small molecules with large dipole moments. 

A further complicating effect in the problem of solubility is that not all compounds decompose into ions on solution. A typical example of such a compound is HgCl2, and obviously the energy is lower in a solution containing undissociated molecules. The energy change in the solution of a compound to undissociated molecules is 

Now (H+ + H ) is unquestionably much greater than if, so that 0M can only be greater than 0 + if S is small. Thus only those 

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