Critical temperature

The critical temperature is that temperature above which unique liquid and gas phases do not exist. As you approach the critical temperature, the properties of the gas and liquid phases become the same, so above the critical temperature there is only one phase. The critical pressure refers to the vapor pressure at the critical temperature. The critical molar volume is the volume of one mole of material at the critical temperature and pressure.

Critical properties vary from material to material, just as is the case for the melting point and boiling point. Critical properties for many pure substances are readily available in the literature. Obtaining critical properties for mixtures is somewhat more problematic.

For the case of pure substances, there is an inflexion point in the critical isotherm on a PV diagram. This means that at the critical point:

(dP/dV)_T = (d²P/dV²)_T = 0

This relation can be used to evaluate two parameters for an equation of state in terms of the critical properties.

Sometimes a set of reduced properties are defined in terms of the critical properties, ie.:

T_r = T/T_c P_r = P/P_c V_r = V/V_c

The principle of corresponding states, indicates that substances at equal reduced pressures and temperatures have equal reduced volumes. This relationship is approximately true for many substances, but becomes increasingly inaccurate for large values of P_r

Two immiscible liquids, such as oil and water, will also have a critical temperature and pressure at which the two phases will become consolute.