acid–base reaction, a type of chemical process typified by the exchange of one or more hydrogen ions, H⁺, between species that may be neutral (molecules, such as water, H₂O; or acetic acid, CH₃CO₂H) or electrically charged (ions, such as ammonium, NH₄⁺; hydroxide, OH⁻; or carbonate, CO₃²⁻). It also includes analogous behaviour of molecules and ions that are acidic but do not donate hydrogen ions (aluminum chloride, AlCl₃, and the silver ion AG⁺).

Acids are chemical compounds that show, in water solution, a sharp taste, a corrosive action on metals, and the ability to turn certain blue vegetable dyes red. Bases are chemical compounds that, in solution, are soapy to the touch and turn red vegetable dyes blue. When mixed, acids and bases neutralize one another and produce salts, substances with a salty taste and none of the characteristic properties of either acids or bases.

The idea that some substances are acids whereas others are bases is almost as old as chemistry, and the terms acid, base, and salt occur very early in the writings of the medieval alchemists. Acids were probably the first of these to be recognized, apparently because of their sour taste. The English word acid, the French acide, the German Säure, and the Russian kislota are all derived from words meaning sour (Latin acidus, German sauer, Old Norse sūur, and Russian kisly). Other properties associated at an early date with acids were their solvent, or corrosive, action; their effect on vegetable dyes; and the effervescence resulting when they were applied to chalk (production of bubbles of carbon dioxide gas). Bases (or alkalies) were characterized mainly by their ability to neutralize acids and form salts, the latter being typified rather loosely as crystalline substances soluble in water and having a saline taste.

In spite of their imprecise nature, these ideas served to correlate a considerable range of qualitative observations, and many of the commonest chemical materials that early chemists encountered could be classified as acids (hydrochloric, sulfuric, nitric, and carbonic acids), bases (soda, potash, lime, ammonia), or salts (common salt, sal ammoniac, saltpetre, alum, borax). The absence of any apparent physical basis for the phenomena concerned made it difficult to make quantitative progress in understanding acid–base behaviour, but the ability of a fixed quantity of acid to neutralize a fixed quantity of base was one of the earliest examples of chemical equivalence: the idea that a certain measure of one substance is in some chemical sense equal to a different amount of a second substance. In addition, it was found quite early that one acid could be displaced from a salt with another acid, and this made it possible to arrange acids in an approximate order of strength. It also soon became clear that many of these displacements could take place in either direction according to experimental conditions. This phenomenon suggested that acid–base reactions are reversible—that is, that the products of the reaction can interact to regenerate the starting material. It also introduced the concept of equilibrium to acid–base chemistry: this concept states that reversible chemical reactions reach a point of balance, or equilibrium, at which the starting materials and the products are each regenerated by one of the two reactions as rapidly as they are consumed by the other.

Apart from their theoretical interest, acids and bases play a large part in industrial chemistry and in everyday life. Sulfuric acid and sodium hydroxide are among the products manufactured in largest amounts by the chemical industry, and a large percentage of chemical processes involve acids or bases as reactants or as catalysts. Almost every biological chemical process is closely bound up with acid–base equilibria in the cell, or in the organism as a whole, and the acidity or alkalinity of the soil and water are of great importance for the plants or animals living in them. Both the ideas and the terminology of acid–base chemistry have permeated daily life, and the term salt is especially common.