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⁺) and a negative ion. Dissociation is the opposite of association or recombination.

For reversible dissociations in a chemical equilibrium

the dissociation constant K_d is the ratio of dissociated to undissociated compound

where the brackets denote the equilibrium concentrations of the species.

The dissociation degree ${\displaystyle \alpha }$ is the fraction of original solute molecules that have dissociated. It is usually indicated by the Greek symbol α. More accurately, degree of dissociation refers to the amount of solute dissociated into ions or radicals per mole. In case of very strong acids and bases, degree of dissociation will be close to 1. Less powerful acids and bases will have lesser degree of dissociation. There is a simple relationship between this parameter and the van 't Hoff factor ${\displaystyle i}$. If the solute substance dissociates into ${\displaystyle n}$ ions, then

For instance, for the following dissociation

As ${\displaystyle n=2}$, we would have that ${\displaystyle i=1+\alpha }$.

The dissociation of salts by solvation in a solution, such as water, means the separation of the anions and cations. The salt can be recovered by evaporation of the solvent.