In the Arrhenius theory, acids are defined as substances that dissociate in aqueous solutions to give H (hydrogen ions or protons), while bases are defined as substances that dissociate in aqueous solutions to give OH (hydroxide ions).
In 1923 physical chemists Johannes Nicolaus Brønsted in Denmark and Thomas Martin Lowry in England both independently proposed the theory named after them. In the Brønsted–Lowry theory acids and bases are defined by the way they react with each other, generalising them. This is best illustrated by an equilibrium equation.
: acid + base ⇌ conjugate base + conjugate acid.
With an acid, HA, the equation can be written symbolically as:
The equilibrium sign, ⇌, is used because the reaction can occur in both forward and backward directions (is reversible). The acid, HA, is a proton donor which can lose a proton to become its conjugate base, A. The base, B, is a proton acceptor which can become its conjugate acid, HB. Most acid–base reactions are fast, so the substances in the reaction are usually in dynamic equilibrium with each other.