Pressure-Volume Diagram for a Vapor-Liquid Mixture

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This Demonstration shows the phase behavior of 1 mol of water on a log pressure versus log volume (-) phase diagram.


Use the sliders to change the specific volume and add heat at constant volume. A piston-cylinder on the right represents the volumes of liquid (blue) and vapor (green) present. The temperature, vapor quality and pressure are shown on the right. The critical point (orange), which is the highest temperature and pressure where two phases coexist, is at 647 K and 22.1 MPa for water. As heat is applied, if the volume selected is smaller than the critical volume, the vapor phase volume decreases; if the volume selected is larger than the critical volume, the liquid phase volume decreases.


Contributed by: Adam J. Johnston and Rachael L. Baumann (April 2019)
Additional contributions by: John L. Falconer
(University of Colorado Boulder, Department of Chemical and Biological Engineering)
Open content licensed under CC BY-NC-SA



Mass and energy balances are used to calculate the pressure and quality when two phases are present:




is internal energy (J/mol) at (the initial condition),

is heat added (J),

is vapor quality,

is internal energy (J/mol) at the final condition,

is volume (),

and the superscripts and refer to liquid and vapor.

The internal energy at pressure is



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