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.

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.

Relation of acids to bases.

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.

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 Common acids and bases and tests therefor two substances are dissolved in water, and the solutions mixed, the chemical action is as follows:

Formation of common salt.

HCL + NaOH =H20 + NaCl

Hydrochloric Caustic soda Common salt acid + (Sodium = Water + (Sodium (Muriatic acid) hydroxid) chloride)

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 C2H402) and soda, or sour milk (lactic acid C3H603) and soda. When bread is "sour," we mean that there was not enough soda to neutralize the acid.

Common examples of neutralization.