a chemical reaction in which a substance decomposes reversibly when the temperature is raised.

A substance undergoing thermal dissociation may form several substances (2H₂O ⇄ 2H₂ + O₂, CaCO₃ ⇄ CaO + CO₂) or a single, simpler substance (N₂O₄ ⇄ 2NO₂, Cl₂ ⇄ 2C1). In thermal dissociation, equilibrium is attained in accordance with the law of mass action and may be characterized by either the equilibrium constant or the degree of dissociation, which is the ratio of the number of dissociated molecules to the total number of molecules. In most cases, thermal dissociation is accompanied by an absorption of heat; that is, the increment of enthalpy is greater than zero (ΔH > 0). According to Le Châtelier’s principle, therefore, heating intensifies thermal dissociation, and the extent to which the equilibrium shifts with temperature is determined by the absolute value of ΔH. Pressure inhibits the thermal dissociation process, which is accompanied by a change Δn in the number of moles of gases; this inhibition becomes stronger as the increase in Δn becomes larger. When Δn = 0, as in the reaction 2HI ⇄ H₂ + I₂, the degree of dissociation is independent of pressure.

The thermal dissociation pressure is uniquely determined by temperature in the case of solids that do not form solid solutions and are not in highly dispersed states. In order to carry out the thermal dissociation of such solids as oxides and crystal hydrates, it is important to know the temperature at which the dissociation pressure becomes equal to the external pressure, in particular, atmospheric pressure. Since the evolved gas may overcome the pressure of the ambient medium, the decomposition process is sharply accelerated when this temperature is reached.

The most useful of the many thermal dissociation processes are the homogeneous reactions in which H₂O and CO₂ are decomposed and certain hydrocarbons are dehydrogenated, and the heterogeneous reactions in which carbonates and sulfides are dissociated. These dissociation reactions are encountered in many heat engineering, chemical, and metallurgical processes, particularly the calcination of limestone, the production of cements, and processes occurring in blast furnaces.