An acid, according to Arrhenius' definition, is a substance that, when dissolved in water, releases hydrogen ions (H+). A base, on the other hand, is a substance that, when dissolved in water, releases hydroxide ions (OH-). When an acid and a base react, the hydrogen ions from the acid combine with the hydroxide ions from the base to form water molecules (H2O).

The overall equation for an acid-base reaction according to Arrhenius' definition is:

Acid + Base → Salt + Water

For example, when hydrochloric acid (HCl) and sodium hydroxide (NaOH) are mixed, the following reaction occurs:

HCl + NaOH → NaCl + H2O

In this reaction, the hydrogen ions from HCl combine with the hydroxide ions from NaOH to form water molecules. The remaining sodium and chloride ions form the salt sodium chloride (NaCl).

Acid-base reactions are important in many biological and chemical processes. For example, the digestion of food in the stomach involves an acid-base reaction between hydrochloric acid and the proteins in food. The neutralization of acids by bases is also used in a variety of industrial and environmental applications.