Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner. For instance, when an acid dissolves in water, a covalent bond between an electronegative atom and a hydrogen atom is broken by heterolytic fission, which gives a proton (H<sup>+</sup>) and a negative ion. Dissociation is the opposite of association or recombination.
Dissociation constant
For reversible dissociations in a chemical equilibrium
:<chem>AB <=> A + B</chem>
the dissociation constant K<sub>d</sub> is the ratio of dissociated to undissociated compound
:<math>K_d = \mathrm{\frac{[A] [B]}{[AB]</math>
where the brackets denote the equilibrium concentrations of the species.
K<sub>a</sub> is variously named a dissociation constant, an acid ionization constant, or an ionization constant.