A phase diagram is common way to represent the various phases of a substance and the conditions under which each phase exists. A phase diagram is a plot of pressure (P or ln P) vs temperature (T). Lines on the diagram represent conditions (T,P) under which a phase change is at equilibrium. That is, at a point on a line, it is possible for two (or three) phases to coexist at equilibrium. In other regions of the plot, only one phase exists at equilibrium.
The phase diagram for a substance is shown below. This particular substance exists in a single form as a solid. A purple dot on the phase diagram marks the current state (temperature and pressure).
The cylinder at the lower right has a movable barrier and contains a pure sample of the substance. The color of the substance indicates its phase: green 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 its density.
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). A single click on a button produces a very small change in conditions. Holding down the mouse button allows the rate of change to accelerate.