A compound that reacts with a protonic acid to give water (and a salt). The definition comes from the Arrhenius theory of acids and bases. Typically, bases are metal oxides, hydroxides, or compounds (such as ammonia) that give hydroxide ions in aqueous solution. Thus, a base may be either:

1. (1) An insoluble oxide or hydroxide that reacts with an acid, e.g.

   $$\text{CuO}(\text{s})+2\text{HCl}(\text{aq})\to {\text{CuCl}}_2(\text{aq})+{\text{H}}_2\text{O}(\text{l})$$

   Here the reaction involves hydrogen ions from the acid

   $${\text{CuO}(\text{s})+2\text{H}}^{+}(\text{aq})\to {\text{H}}_2\text{O}(\text{l})+{\text{Cu}}^{2+}(\text{aq})$$

2. (2) A soluble hydroxide, in which case the solution contains hydroxide ions. The reaction with acids is a reaction between hydrogen ions and hydroxide ions:

   $${\text{H}}^{+}{+\text{OH}}^{-}\to {\text{H}}_2\text{O}$$

3. (3) A compound that dissolves in water to produce hydroxide ions. For example, ammonia reacts as follows:

   $${\text{NH}}_3(\text{g})+{\text{H}}_2\text{O}(\text{l})\rightleftharpoons {\text{NH}}_4^{+}{(\text{aq})+}^{-}\text{OH}$$

   Similar reactions occur with organic amines (see also nitrogenous base; amine salts). A base that dissolves in water to give hydroxide ions is called an alkali. Ammonia and sodium hydroxide are common examples.

The original Arrhenius definition of a base has been extended by the Lowry-Brønsted theory and by the Lewis theory. See acid.