The scientist Svante Arrhenius first proposed that acids dissociate in water to form ions. According to him, acids were materials that included a hydrogen ion. Dissolved in water, the hydrogen ion, H⁺, gives the solution the characteristics of an acid. Arrhenius also developed a corresponding definition for a base. When dissolved in water, bases produce hydroxide ions, OH^-, that give the solution the characteristics of a base.

The Arrhenius definitions cover many of the most common acids and bases and their chemical reactions, but there are other materials that have the characteristics of acids but don't fit the Arrhenius definition. Broader definitions of acids can include some of these materials.

Historically, acids were described as sour and corrosive, but little was known about the basis for these characteristics. In 1884, Svante Arrhenius proposed that compounds such as NaCl or table salt formed charged particles called ions when they dissolved in water. By 1887, Arrhenius had developed a theory that led him to suggest that acids ionized in water to produce hydrogen ions. The hydrogen ions gave acids their characteristics.

An important characteristic of acids is that they react with metals to form a salt and hydrogen gas. Using the Arrhenius definition of an acid, it is clear that the acid dissolves in water into hydrogen ions and the other negative ions from the acid. The metal combines with the negative ions, leaving the hydrogen ions and extra electrons to form hydrogen gas.

Acids also react with bases to form a salt and water. According to the Arrhenius definition, bases produce hydroxide ions in solution. As a result, in an acid-base reaction, the hydrogen ions from the acid combine with the hydroxide ions from the base to form water molecules. The negative ions from the acid combine with the positive ions from the base to form a salt.

When a typical Arrhenius acid such as hydrochloric acid reacts with a metal or a base, the Arrhenius definitions make it easy to follow the reactions. For example, hydrochloric acid, HCl, reacts with zinc, Zn, to form zinc chloride and hydrogen gas. The negative Cl ions combine with the zinc atoms to form the ZnCl₂ molecules and generate extra electrons. The electrons combine with the hydrogen ions from the acid to become hydrogen gas. The chemical formula is Zn + 2HCl = ZnCl₂ + H₂.

When hydrochloric acid combines with a base such as sodium hydroxide, NaOH, the base dissociates into sodium and hydroxide ions. The hydrogen ions from the hydrochloric acid combine with the hydroxide ions from the sodium hydroxide to form water. The sodium ions combine with the chlorine ions to form NaCl or table salt. The chemical formula is HCl + NaOH = NaCl + H₂O.

The Arrhenius definition of acids is narrow in the sense that it only applies to substances that dissolve in water and only to those that have hydrogen ions. A broader definition defines acids as substances that increase the concentration of hydrogen ions when dissolved in water.

Even broader definitions such as the Lewis or the Bronsted-Lowry definitions describe acids as electron acceptors or as proton donors. They include substances that display the characteristics of acids but don't fit the traditional definition. For common chemistry reactions on the other hand, the Arrhenius definitions form a good basis for explaining how the reactions work.