Critical Point

Critical Point, in physics, point on the temperature or pressure scale, which marks a change in the physical state of a substance. The critical point of a metal alloy is the temperature during the cooling of the substance at which a molecular rearrangement takes place, giving rise to a different form of the substance, usually with the absorption or evolution of heat. The critical temperature of a gas is the maximum temperature at which the gas can be liquefied; the critical pressure is the pressure necessary to liquefy the gas at the critical temperature. Some gases, such as helium, hydrogen, and nitrogen, have low critical temperatures and require intensive cooling before they can be liquefied. Others, such as ammonia and chlorine, have high critical temperatures and can be liquefied at ordinary room temperature by pressure alone. The accompanying table shows critical temperatures and pressures for representative gases.

A third description of the critical point is the critical volume. This is the volume that one mole of gas would occupy at its critical temperature and pressure. These three quantities: critical temperature, pressure, and volume are called, collectively, the critical constants of a substance.