Acid

Acids are generally defined as those chemical substances which release hydrogen ions on dissolving in water. Although acids are most often thought of as proton (H⁺) or hydronium ion ([H₃O]⁺) donors, as defined by the Brønstad-Lowry theory, the Lewis acid-base theory is more comprehensive and defines acids as electron pair acceptors. As such, acids are the opposite of bases in chemistry. Acids dissolved in otherwise pure water have a pH less than 7 at 25°C. Compounds, substances, or materials which overall have the nature of an acid can be called acidic; for example, aqueous solutions (solutions of water) having a pH less than 7 can be considered acidic.

Acids play an integral role in the metabolic functions of animals, in chemistry, material science, and the food industry. Acids, such as the twenty common amino acids and carboxylic acids (including valeric acid, and propionic acid), are necessary for life.

A distinction is made between strong and weak acids. Strong acids, e.g. hydrochloric acid (HCl), nitric acid (HNO₃), are completely dissociated in a solution. This means for solutions of equivalent concentration, there will be a higher concentration of H⁺ ions for a strong acid than a weak acid. Weak acids, e.g. ethanoic acid or acetic acid (CH₃COOH), do not dissociate completely but release only a small fraction of their hydrogens as H⁺ ions. There are also intermediate strength acids.

Brønstad-Lowry acids

According to the theory proposed by Brønstad and Lowry, an acid is defined as a species which donates a proton (H⁺ ion) or a hydronium ([H₃O]⁺) ion. For example, hydrochloric acid (HCl) donates a H⁺ ion. Therefore it is an acid. Sodium hydroxide (NaOH), by contrast, donates a OH^- ion, and so, it is not considered an acid. Not all hydrogen atoms in a molecule will dissociate (leave the molecule) as H⁺ or H₃O⁺ ions, so those hydrogen atoms are not considered acidic. For example, hydrogens on the -CH₃ group of acetic acid practically will not come off as H⁺ ions, and therefore only the hydrogen on the -COOH group is considered acidic.
There is a drawback in this theory: acids which do not release protons or hydronium ions are not considered acids. . For example, in the following reaction, boron trifluoride does not donate a proton or hydronium ion, so it is not a Bronstad-Lowry acid.

$BF_{3}\;+\;:NH_{3}\;\longrightarrow \;F_{3}B:NH_{3}$

Yet by nature BF₃ is an acid.
Consequently, this definition of acids has been discarded in favor of Lewis' definition.

Lewis acids

According to the Lewis definition, an acid is a compound that accepts an electron pair. Since the compound BF₃ can accept or combine with the electron pair of a compound like NH₃, BF₃ is considered an acid by the Lewis definition. The Lewis definition encompasses the Arrhenius and Bronstad-Lowry definitions of an acid.

For a discussion of Lewis acids and Lewis bases, see Lewis acid-base theory.