on tastes sour. (4) It has the power to change the
color of certain substances called _indicators_. Thus blue litmus is
changed to red, and yellow methyl orange is changed to red. Since all
acids produce hydrogen cations, while the anions of each are different,
the properties which all acids have in common when in solution, such as
taste and action on indicators, must be attributed to the hydrogen ions.

DEFINITION: _An acid is a substance which produces hydrogen ions when
dissolved in water or other dissociating liquids._

~Undissociated acids.~ When acids are perfectly free from water, or are
dissolved in liquids like benzene which do not have the power of
dissociating them into ions, they should have no real acid properties.
This is found to be the case. Under these circumstances they do not
affect the color of indicators or have any of the properties
characteristic of acids.

The familiar bases. The bases most used in the laboratory are sodium
hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide
(Ca(OH)_{2}). These are white solids, soluble in water, the latter
sparingly so. Some bases are very difficultly soluble in water. The very
soluble ones with most pronounced basic properties are sometimes called
the _alkalis_.

~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 aci