<P> In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium . For example, the triple point of mercury occurs at a temperature of − 38.83440 ° C and a pressure of 0.2 m Pa . </P> <P> In addition to the triple point for solid, liquid, and gas phases, a triple point may involve more than one solid phase, for substances with multiple polymorphs . Helium - 4 is a special case that presents a triple point involving two different fluid phases (lambda point). </P>

The temperature and pressure at which a liquid and a gas are in equilibrium