Two Components: Liquid–Liquid Systems

The pressure-composition phase diagram for an ideal solution of two volatile liquids displays two lines: the bubble point line, representing a plot of the pressure versus the liquid mole fraction, and the dew point line, plotting pressure versus vapor mole fraction.

A tie line is a horizontal line connecting liquid and vapor phase compositions in the phase diagram.

For a liquid with a known composition, the equilibrium vapor pressure is found by tracing a path upwards on the diagram until it intersects with the bubble point line.

At that equilibrium pressure, the vapor composition is measured by horizontally reaching the dew point line.

Similarly, temperature-composition diagrams display the equilibrium phase compositions at different temperatures.

Here, the 'liquid' line shows the boiling temperature for different compositions, while the 'vapor' line displays the vapor composition in equilibrium with the liquid at each temperature.

The point below the liquid line at a given temperature shows that the substance is fully liquid, while above the vapor line, it is completely gaseous.

In addition, an 'isopleth' is a vertical line that represents the constant composition of a system as pressure and temperature change in a direct relationship.