Chemists use several different definitions of acids, as follows.


Arrhenius theory

According to the Arrhenius theory, an acid is a substance that contains hydrogen and dissociates in water to produce positive hydrogen ions, i.e., protons. Such acids tend to be corrosive substances with a sharp taste, which turn litmus red and give color changes with other indicators. They are referred to as protonic acids and are classified into strong acids, which are almost completely dissociated in water (e.g., sulfuric acid, nitric acid, and hydrochloric acid), and weak acids, which are only partially dissociated (e.g., acetic acid and hydrogen sulfide). The strength of an acid depends on the extent to which it dissociates, and is measured by its dissociation constant. Acids have pH values ranging from 1 (for a strong acid) to just under 7 (for a very weak acid). Compare with alkali.


Lowry-Brønsted theory (1923)

In this, the definition is extended to one in which an acid is a proton donor (a Brønsted acid), and a base is a proton acceptor (a Brønsted base). An important feature of the Lowry-Brønsted concept is that when an acid gives up a proton, a conjugate base is formed that is capable of accepting a proton. Similarly, every base produces its conjugate acid as a result of accepting a proton. For example, the acetate ion is the conjugate base of acetic acid, and the ammonium ion is the conjugate acid of ammonia. As the acid of a conjugate acid/base pair becomes weaker, its conjugate base becomes stronger and vice versa.


Lewis theory

A further extension of the idea of acids and bases. A Lewis acid is a compound or atom that can accept a pair of electrons and a Lewis base is one that can donate an electron pair. This definition encompasses "traditional" acid-base reactions, but it also includes reactions that do not involve ions, e.g. H3N: BCl3 → H3NBCl3 in which NH3 is the base (donor) and BCl3 the acid (acceptor).


Occurrence and uses of acids

Many chemical reactions are speeded up in acid solution, giving rise to important industrial applications (acid-base catalysis). Mineral acids, including sulfuric acid, nitric acid, and hydrochloric acid, find widespread use in industry.


Organic acids, which occur widely in nature, tend to be weaker. Carboxylic acids (including acetic acid and oxalic acid) contain the acidic group –COOH; aromatic systems with attached hydroxyl group (phenols) are often also acidic. Amino acids, constitutive of proteins, are essential components of all living systems on Earth.