In chemistry, a base is most commonly thought of as an aqueous substance that can accept hydrogen ions. A soluble base is also often referred to as an alkali if OH− ions are involved. This refers to the Brønsted-Lowry theory of acids and bases. Alternate definitions of bases include electron pair donors (Lewis), as sources of hydroxide anions (Arrhenius). In addition to this, bases can commonly be thought of as any chemical compound that, when dissolved in water, gives a solution with a pH higher than 7.0. Examples of simple bases are sodium hydroxide and ammonia.

Bases can be thought of as the chemical opposite of acids. A reaction between an acid and base is called neutralization. Bases and acids are seen as opposites because the effect of an acid is to increase the hydronium ion (H3O+) concentration in water, whereas bases reduce this concentration. Bases react with acids to produce water and salts (or their solutions).

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