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.