the resulting H^{+}
and OH^{-} ions are present only in minute concentrations (1 mol. of
dissociated water in 10^{7} liters), yet under some conditions they
may give rise to important consequences. The term !hydrolysis! is
applied to the changes which result from the reaction of these ions.
Any salt which is derived from a weak base or a weak acid (or both)
is subject to hydrolytic action. Potassium cyanide, for example, when
dissolved in water gives an alkaline solution because some of the
H^{+} ions from the water unite with CN^{-} ions to form (HCN), which
is a very weak acid, and is but very slightly dissociated. Potassium
hydroxide, which might form from the OH^{-} ions, is so largely
dissociated that the OH^{-} ions remain as such in the solution. The
union of the H^{+} ions with the CN^{-} ions to form the undissociated
HCN diminishes the concentration of the H^{+} ions, and more water
dissociates (H_{2}O <--> H^{+} + OH^{-}) to restore the equilibrium.
It is clear, however, that there must be a gradual accumulation of
OH^{-} ions in the solution as a result of these changes, causing the
solution to exhibit an alkaline reaction, and also that ultimately the
further dissociation of the water will be checked by the presence of
these ions, just as the dissociation of the H_{2}S was lessened by the
addition of HCl.

An exactly opposite result follows the solution of such a salt as
Al_{2}(SO_{4})_{3} in water. In this case the acid is strong and the
base weak, and the OH^{-} ions form the little dissociated Al(OH)_{3},
while the H^{+} ions remain as such in the solution, sulphuric acid
being extensively dissociated. The solution exhibits an acid reaction.

Such hydrolytic processes as the above are of great importance in
analytical chemistry, especially in the understanding of the action of
indicators in volumetric analysis. (See page 32.)

The impelling force which causes an element to pass from the atomic
to the ionic condition is termed !electrolytic solution pressure!, or
ionization tension. This force may be measured in terms of electrical
potential, and the table below shows the relative values for a number
of elements.

In general, an element with a greater solution pressure tends to cause
the deposition of an element of less solution pressure when placed in
a solution of its salt, as, for instance, when a strip of zinc or
iron is placed in a solution of a copper salt, with the resulting
precipitation of