[Sidenote: Relation of acids to bases]
We have already discussed a number of substances called acids. It is
necessary to inquire why chemists call them acids. What is there in
common, for example, between the heavy, oily liquid sulfuric acid and
the colorless gas, hydrochloric acid? It is not possible to understand
the nature of their common properties without examining a class of
substances called alkalis or bases.
Acids and bases have the power to destroy the characteristic properties
of each other. When an acid is brought into contact with a base, in
proper proportions, the characteristic properties of both the acid and
the base are destroyed. They are said to neutralize each other.
[Sidenote: Common acids and bases and tests therefor]
The most common acids are sulfuric, hydrochloric, and nitric. Among
the more common bases are caustic soda, caustic potash, and lime. A
convenient way to recognize whether a substance has acid or basic
properties is by means of certain color-changes. Litmus is a coloring
matter which is ordinarily blue. If a solution which is colored blue
with litmus be treated with a drop or two of an acid, the color is
changed to red. If the red solution be treated with a few drops of a
solution of a base, the blue color is restored.
Many substances change in color according to whether the solutions in
which they are present are acid or alkaline. An infusion of red cabbage,
for example, changes color when treated with an acid, and recovers its
color when again treated with an alkali.
[Sidenote: Formation of common salt]
What happens in the chemical sense in this neutralizing process is
nicely illustrated by the formation of common salt from hydrochloric
acid and caustic soda, also called sodium hydroxid. When these two
substances are dissolved in water, and the solutions mixed, the chemical
action is as follows:
HCL + NaOH = H₂O + NaCl
Hydrochloric acid + Caustic soda = Water + Common salt
(Muriatic acid) (Sodium hydroxid) (Sodium chloride)
[Sidenote: Common examples of neutralization]
The strong hydrochloric acid with its pungent odor and sour taste, and
the caustic alkali with its equally characteristic properties have both
disappeared, and in their place we find nothing more wonderful than
common salt dissolved in water. Other forms of neutralization that are
very common are vinegar (acetic acid C₂H₄O₂) and soda, or sour milk
(lactic acid C₃H₆O₃) and soda. When bread is "sour," we mean that there
was not enough soda to neutralize the acid.