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ION–SOLVENT INTERACTIONS 37

energetically than they have within the lattice. It entices them out of the lattice and
into the solution. 2
Of course, this implies that there is a considerable energy of interaction between
the lattice ions and the solvent molecules. It is this ion–solvent interaction, the
immediate cause of the formation of conducting ionic solutions from salts, that is the
subject of this chapter.

2.2. BREADTH OF SOLVATION AS A FIELD
The wide range of areas affected by solvation can be seen when one considers the
basic role hydration plays in; for example, geochemistry, and indeed in the whole
hydrosphere. The pH of natural waters, with all the associated biological effects, is
affected by the dissolution of from river beds; and the degree of this dissolution,
like any other, is determined by the solvation of the ions concerned. Alternatively,
consider the modern environmental problem of acid rain. The basic cause is the
formation of atmospheric as a result of burning fossil fuels. The pH reached in
naturally occurring water is a result of the dissolution of in rain and the subsequent
creation of the sulfuric acid; because the stability of the and ions that
arise is determined by their hydration. The acidity of natural waters then depends upon
the original concentration of in the air as well as the action of various associated
ionic reactions which tend to counter the pH change the causes, but which, because
they involve ions, themselves depend for their energetics on the ions’ solvation
energies.
It has already been implied that ion–solvent interactions have widespread signifi-
cance in electrochemistry, and some of the ramifications of this were discussed in

2
In this process (and up to a certain concentration) all the ions in the lattice salt become mobile solution
ions, although when the ionic concentration gets high enough, the negative and positive ions (anions and
cations, respectively) start to associate into nonconducting ion pairs. The specific conductance of salt
solutions therefore passes through a maximum, if plotted against concentration.
3
In this chapter, solvation and hydration will both be used to describe the interaction of an ion with its
surroundings. Clearly, solvation is the general term but most cases of it are in fact hydration.