Phase Diagram

Concepts

A phase diagram is common way to represent the various phases of a substance and the conditions under which each phase exists.

The phase diagram shown at the bottom of this page is a plot of pressure ( ln P , to be specific) vs temperature (T ). Lines on the diagram represent conditions ( T , P ) under which two phases can coexist at equilibrium. At any point in the plot that does not lie on a phase line, only one phase exists at equilibrium.

Triple Point
At the triple-point ( temperature T₃ and pressure P₃ ), three phases can coexist at equilibrium.

Critical Point
At a temperature above the critical temperature, T_c , and a pressure above the critical pressure, P_c , it is no longer possible to distinguish between the gas and liquid phases. At T > T_c and P > P_c the substance is referred to as a super-critical fluid.

Interactive Phase Diagram

Each substance has an unique phase diagram. An example of a phase diagram is shown below. This particular substance exists in a single form as a solid.

A blue dot on the phase diagram marks the current state, which is defined by the temperature and pressure of the substance.

The cylinder at the lower right has a movable barrier and contains a pure sample of the substance (and nothing else). The color of the substance indicates its phase: yellow for the solid phase, blue for the liquid phase, and red for the gas phase. For the liquid and gas phases, the shade of the color is related to the density of the substance.

The controls allow the user to heat or cool the sample (thereby changing the temperature) and compress or expand the sample (thereby changing the pressure).

The simulation only accepts temperatures between 100. and 500. K and pressures between 1 and 400. atm.