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.

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.

Brønstad-Lowry acids

According to the theory proposed by Brønstad and Lowry, an acid is defined as a substance which donates a proton (H⁺ ion) or a hydronium (H₃O⁺) ion on dissolving in water. For example, Hydrochloric Acid(HCl) on dissolving in water releases a H⁺ ion. Therefore it is an acid. Sodium Hydroxide (NaOH), by contrast, releases a OH^- ion on dissolving in water, and so, it is not considered an acid. There is a drawback in this theory: acids which do not release protons or hydronium ions are not considered acids. Consequently, this definition of acids has been discarded in favor of Lewis' definition.