dissociation, in chemistry, separation of a substance into atoms or ions. Thermal dissociation occurs at high temperatures. For example, hydrogen molecules (H₂) dissociate into atoms (H) at very high temperatures; at 5,000°K; about 95% of the molecules in a sample of hydrogen are dissociated into atoms. Electrolytic dissociation occurs when an electrolyte is dissolved in a polar solvent. For example, when hydrogen chloride, HCl, is dissolved in water to form hydrochloric acid, most of its molecules dissociate into hydrogen ions (H⁺) and chloride ions (Cl^-). Some pure substances spontaneously dissociate. For example, in pure water some of the molecules dissociate to form hydrogen ions and hydroxyl ions. Dissociation is generally reversible; when the atoms or ions of the dissociated substance are returned to the original conditions, they recombine in the original form of the substance. The dissociation constant is a measure of the extent of dissociation. It is represented by the symbol K. In the simplest case, if a substance AB dissociates into two parts A and B and the concentrations of AB, A, and B are represented by [AB], [A], and [B], then K=[A]×[B]/[AB]. The dissociation constant is measured at equilibrium, and its value is usually affected by changes in temperature.

Breaking of a chemical compound into simpler constituents as a result of added energy, as in the case of gaseous molecules dissociated by heating; also, the effect of a solvent on a dissolved polar compound (electrolyte), as in the case of an inorganic salt, such as sodium chloride, dissolved in water. All electrolytes dissociate into ions to a greater or lesser extent in polar solvents (in which the molecules are electric dipoles). The degree of dissociation can be used to determine the equilibrium constant. Dissociation is used to explain electrical conductivity and many other properties of electrolytic solutions.

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