A phase diagram is a graphical way of showing the physical state of a pure substance at varying pressures and temperatures. As seen in the image to the right, a phase diagram is typically represented as a graph with temperature on the x-axis and pressure on the y-axis.
A phase diagram will always have a number of lines on it, representing a phase equilibrium, that is, the set of temperatures and pressures at which one phase is in equilibrium with another phase. These lines are called phase boundaries, and they section the area of the plot into the three phases of matter (solid, liquid, and gas) as well as the supercritical fluid region.
Along with the phase boundaries, there are two important points on a phase diagram: the triple point and the critical point. The triple point is the unique temperature and pressure at which all three phases of matter coexist in equilibrium. The critical point defines the critical temperature (Tc) and critical pressure (Pc) above which the substance is a supercritical supercritical fluid.
Other Types of Phase Diagrams
While the pressure-temperature diagrams (PT diagrams) are useful for discerning the phase of a substance at certain operating conditions, other types of phase diagrams exist. For example, there are phase diagrams which describe the phase of a mixture of two compounds verses temperature. Metallurgists also use a special type of phase diagram to describe solid-solid alloys, which have different "phases," or different ways the molecules crystalize depending on the composition of the alloy at the temperature at which it is held.