ISOCHORE, in physics, a line on a thermodynamic diagram showing certain relations of pressure and temperature in a gas. If a gas is kept at a constant volume (e.g., in a closed vessel) while the temperature to which it is subject is varied, the pressure exerted on the walls of the vessel by the gas alters in a definite manner. If, further, the way in which the temperature and pressure are thus related is indicated by a line on a graph, this line is known as an isochore (Lao, equal; xopos, space).
The term is often applied also to van't Hoff's "reaction iso chore," which may be briefly described. If a reversible chemical
reaction takes place at a constant volume, the equilibrium con stant, which determines the extent to which the reaction proceeds (see CHEMICAL ACTION), varies with the temperature in a way which depends upon the heat evolved by the reaction, thus: Here K is the equilibrium constant, T the absolute temperature, and Q the heat (in calories) which would be evolved if I gram equivalent of the initial substance was completely converted to the final product at the temperature T.