Water in this, and similar actions, takes the place of and plays the rôle of, an ‹acid›, and ‹the metal hydroxides or bases appear as its salts›.[364] It is a ‹very weak› acid, which can easily be driven out of its salts by any stronger acid (neutralization of bases), but that does not alter the conclusions reached. Considered from the point of view of the theory of ionization, the relation [p178] would be expressed by saying that in the common bases the positive hydrogen ion of water has been replaced by some other positive or metal ion. The salt of any acid could be defined in exactly the same way.
«Water as a Base.»—Acid oxides, such as carbon dioxide, silicon dioxide, arsenious oxide, combine more or less readily with bases, such as sodium hydroxide, to form salts:
As_{ 2}O_{ 3} + 2 NaOH → 2 NaAsO_{ 2} + H_{ 2}O.
A number of acid oxides combine with water in exactly the same manner, and sometimes with such tremendous vigor, that great care must be taken in bringing the two together, as is the case when sulphur trioxide or phosphorus pentoxide are added to water (‹Exp.›).
It is evident that in such actions water may take the place of, and play the rôle of, an ordinary base, forming the ‹acids›, which may well be defined as hydrogen salts.[365] It is true that the basic properties of water are so weak, that the metal ion of even a weak base, like ammonium hydroxide, will replace the hydrogen-ion in its salts, the acids, quite readily (HCl + NH_{ 4}OH ⥂ NH_{ 4}Cl + H_{ 2}O). But such a weak base, in turn, will have to give way, of course, to still stronger bases; for instances, NH_{ 4}Cl + NaOH ⥂ NaCl + NH_{ 4}OH. From the point of view of the theory of ionization, the hydrogen-ion is positive, like all the other metals ions whose hydroxides are bases.
There should be no difficulty, therefore, in considering water to have the chemical properties of a base as well as of an acid. Its chemical activities as such, weak as they may be, must be satisfied whenever it is present. These activities lead to the hydrolysis or the decomposition of salts by water, in greater or lesser degree, whenever water is used as a solvent for salts.
«Action of Water on a Salt of a Strong Base and a Strong Acid.»—If sodium chloride, a typical salt formed from a strong base and a strong acid, is dissolved in water, it is ionized to a considerable extent. Considering the solution from a mechanical point of view, we would expect that the sodium ions, moving in all directions, would collide occasionally with hydroxide ions, which are formed from the water and are present in minute but definite quantity. Some of the collisions must result in the formation of sodium hydroxide, as we have no reason to suppose that the result would differ from that in other cases where positively charged particles meet with negatively charged ones. However, since sodium hydroxide is an ionogen, with a very great tendency to ionize, and since there is present only a minute concentration of the hydroxide-ion, the [p179] equilibrium conditions will be satisfied when only traces of the nonionized hydroxide are formed. In a similar manner, we must expect to have traces, and only traces, of nondissociated hydrochloric acid formed by the union of chloride ions with some of the hydrogen ions of the water. Since hydrogen chloride and sodium hydroxide show practically the same tendency to ionize (tables, pp. 104 and 106), the two kinds of ions which water forms, the hydrogen-ion and the hydroxide-ion, will be used up ‹to a very slight› and practically ‹equal› extent to form nonionized sodium hydroxide and hydrogen chloride, but the ions will be immediately regenerated, and in equal concentrations, from the nonionized water which is present. All the equilibrium requirements will be satisfied when ‹traces› of sodium chloride have been converted into nonionized sodium hydroxide and hydrogen chloride. Such a solution, containing no excess of the hydrogen- or the hydroxide-ion, would react ‹neutral›. The action may be expressed by the equation[366]