A state of Thermodynamic equilibrium between the liquid and gas state.

The PT diagram for Pure Substance will see Vapour Liquid Equilibrium (VLE) as a set of points along a smooth growing curve ending up with a Critical Point $\begin{array}{l}(T_c, p_c)\end{array}$ (see Fig. 1).

The PT diagram for Fluid Mixture will see Vapour Liquid Equilibrium (VLE) as a closed area with an apex of Pseudo-Critical Point $\begin{array}{l}(T_{pc}, p_{pc})\end{array}$ (see Fig. 2).


Fig. 1. Schematic PT diagram with Vapour Liquid Equilibrium curve and Critical Point.	Fig. 2. Schematic PT diagram with Vapour Liquid Equilibrium area and Pseudo-Critical Point.

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