This P-x-y diagram shows the regions of phase stability for a benzene-toluene mixture at 20°C. This mixture is modeled as an ideal solution that obeys Raoult's law. The solid red line is the bubble pressure and represents the liquid mole fraction. The solid blue curve is the dew pressure and represents the vapor composition.

Click and drag the purple dot to adjust the mixture's pressure and mole fraction of the benzene. Two phases only co-exist in equilibrium in the region bounded by the red and blue curves. When two phases co-exist, a horizontal line (the dashed line in the diagram, a tie line) through the purple point intersects the bubble and dew pressure curves at the liquid and vapor compositions, respectively, that are in equilibrium. The mole fractions of the phases are indicated by the vertical dotted lines. The relative amounts of liquid and vapor, which are shown in the bar graph, are calculated using the lever rule. The amount of vapor is proportional to the length of the dashed horizontal blue line and the amount of liquid is proportional to the length of the dashed horizontal red line.

This simulation was created in the Department of Chemical and Biological Engineering at University of Colorado Boulder for LearnChemE.com by Patrick Doyle. It is based on a Mathematica simulation on the Wolfram Demonstration Project website prepared by Lisa Goss and modified by Rachael Baumann. Address any questions or comments to learncheme@gmail.com. All of our simulations are open source, and are available on our LearnChemE Github repository.

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