Brønsted–Lowry theory

In the Brønsted–Lowry acid–base theory, an acid is defined as a proton donor and a base is defined as a proton acceptor. Here, proton means a hydrogen ion, H⁺ (a hydrogen atom without its electron is just a proton).

Some substances, such are water, are amphoteric (or amphiprotic). This means they are capable of acting as an acid or as a base in different chemical reactions. Acids can be classified as monoprotic (having one ionizable proton, like HCl) or polyprotic (having multiple ionizable protons, like the diprotic acid H₂SO₄).

In the following reaction, HCl_(aq) is the acid and H₂O_(l) is the base because the former gives its proton to the latter:

HCl_(aq) + H₂O_(l) → Cl⁻_(aq) + H₃O⁺_(aq).

In the reverse reaction, Cl⁻_(aq) would be the base, so it is called the conjugate base. Similarly, H₃O⁺_(aq) would be the acid in the reverse reaction, so it is the conjugate acid. A conjugate acid–base pair is a pair of substances whose formulas differ by a single proton. In the above reaction, the conjugate acid–base pairs are HCl/Cl⁻ and H₂O/H₃O⁺.