Historically, the first chemical definition of an acid and a base was put forward by Svante Arrhenius, a Swedish chemist, in 1884. An Arrhenius acid is a compound that increases the H⁺ ion concentration in aqueous solution. The H⁺ ion is just a bare proton, and it is rather clear that bare protons are not floating around in an aqueous solution. Instead, chemistry has defined the hydronium ion (H₃O⁺) as the actual chemical species that represents an H⁺ ion. H⁺ ions and H₃O⁺ ions are often considered interchangeable when writing chemical equations (although a properly balanced chemical equation should also include the additional H₂O). Classic Arrhenius acids can be considered ionic compounds in which H⁺ is the cation. Table 12.2 “Some Arrhenius Acids” lists some Arrhenius acids and their names.

An Arrhenius base is a compound that increases the OH⁻ ion concentration in aqueous solution. Ionic compounds of the OH⁻ ion are classic Arrhenius bases.

Acids have some properties in common. They turn litmus, a plant extract, red. They react with some metals to give off H₂ gas. They react with carbonate and hydrogen carbonate salts to give off CO₂ gas. Acids that are ingested typically have a sour, sharp taste. (The name acid comes from the Latin word acidus, meaning “sour.”) Bases also have some properties in common. They are slippery to the touch, turn litmus blue, and have a bitter flavour if ingested.

Acids and bases have another property: they react with each other to make water and an ionic compound called a salt. A salt, in chemistry, is any ionic compound made by combining an acid with a base. A reaction between an acid and a base is called a neutralization reaction and can be represented as follows:

acid + base → H₂O + salt

The stoichiometry of the balanced chemical equation depends on the number of H⁺ ions in the acid and the number of OH⁻ ions in the base.