Immiscible Liquids on Pressure-Composition Diagram

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The pressure-composition phase diagram for two immiscible liquids, benzene and water, is at constant temperature. Set the temperature with a slider to change the saturation pressures. Set the overall benzene mole fraction with a slider. The bar graph shows the moles of liquid water (blue), liquid benzene (orange) and vapor (green), which contains both components. The system contains one mole total. You can change the piston height to change the pressure and the amounts of each phase. At a given temperature, all three phases co-exist at only one pressure. When the piston height increases at this pressure, one of the liquid phases completely evaporates before the pressure decreases. Select "piston-cylinder" instead of "amounts of each phase" to view a representation of a piston-cylinder with the volume of each phase on a log scale.

Contributed by: Rachael L. Baumann (February 2015)
Additional contributions by: John L. Falconer, Derek M. Machalek, Nathan S. Nelson, and Garrison J. Vigil
(University of Colorado Boulder, Department of Chemical and Biological Engineering)
Open content licensed under CC BY-NC-SA



Immiscible components do not mix in the liquid phase, and each exerts its own saturation pressure and when present as a liquid; the subscripts and refer to benzene and water. The total pressure above the two immiscible liquids is equal to the sum of their saturation pressures:


The Antoine equation is used to calculate the saturation pressures:


where , , and are Antoine constants for component , and is temperature (°C).

For the benzene-water system, for conditions where benzene condenses, the dew point curve is:


where is the mole fraction of benzene in the vapor phase.

For conditions where water condenses, the dew point curve is:


where is the mole fraction of water in the vapor phase.

The screencast video at [1] explains how to use this Demonstration.


[1] Immiscible Liquids on a Pressure-Composition Diagram.

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