d and a base are brought together in solution
in proper proportion, the characteristic properties of each disappear.
The solution tastes neither sour nor brackish; it has no effect upon
indicators. There can therefore be neither hydrogen nor hydroxyl ions
present in the solution. A study of reactions of this kind has shown
that the hydrogen ions of the acid combine with the hydroxyl ions of the
base to form molecules of water, water being a substance which is not
appreciably dissociated into ions. This action of an acid on a base is
called _neutralization_. The following equations express the
neutralization of the three acids by three bases, water being formed in
each case.

Na^{+}, OH^{-} + H^{+}, Cl^{-} = Na^{+}, Cl^{-} + H_{2}O.

K^{+}, OH^{-} + H^{+}, NO_{3}^{-} = K^{+}, NO_{3}^{-} + H_{2}O.

Ca^{++}, (OH)_{2}^{--} + H_{2}^{++}, SO_{4}^{--}
= Ca^{++}, SO_{4}^{--} + 2H_{2}O.

DEFINITION: _Neutralization consists in the union of the hydrogen ion of
an acid with the hydroxyl ion of a base to form water._

~Salts.~ It will be noticed that in neutralization the anion of the acid
and the cation of the base are not changed. If, however, the water is
expelled by evaporation, these two ions slowly unite, and when the water
becomes saturated with the substance so produced, it separates in the
form of a solid called a _salt_.

DEFINITION: _A salt is a substance formed by the union of the anion of
an acid with the cation of a base._

~Characteristics of salts.~ (1) From the definition of a salt it will be
seen that there is no element or group of elements which characterize
salts. (2) Salts as a class have no peculiar taste. (3) In the absence
of all other substances they are without action on indicators. (4) When
dissolved in water they form two kinds of ions.

~Heat of neutralization.~ If neutralization is due to the union
of hydrogen ions with hydroxyl ions, and nothing more, it
follows that when a given weight of water is formed in
neutralization, the heat set free should always be the same, no
matter from what acid and base the two kinds of ions have been
supplied. Careful experiments have shown that this is the case,
provided no other reactions take place at the same time. When
18g. of water are formed in neutralization, 13,700 cal. of heat
are set free. This is represented in the equations

Na^{+}, OH^{-} + H^{+}, Cl^{-}
= Na^{+}, C