~Characteristics of bases.~ (1) All bases contain hydrogen and oxygen. (2) When dissolved in water the molecules of the base dissociate into two kinds of ions. One of these is always composed of oxygen and hydrogen and is the anion. It has the formula OH and is called the _hydroxyl ion_. The remainder of the molecule, which usually consists of a single atom, is the cation. (3) The solution of a base has a soapy feel and a brackish taste. (4) It reverses the color change produced in indicators by acids, turning red litmus blue, and red methyl orange yellow. Since all bases produce hydroxyl anions, while the cations of each are different, the properties which all bases have in common when in solution must be due to the hydroxyl ions. DEFINITION: _A base is a substance which produces hydroxyl ions when dissolved in water or other dissociating liquids._ ~Undissociated bases.~ Bases, in the absence of water or when dissolved in liquids which do not dissociate them, should have none of the properties characteristic of this class of substances. This has been found to be the case. For example, they have no effect upon indicators under these circumstances. ~Neutralization.~ When an acid 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._