A phase diagram allows us to see the pressures and temperatures a particular phase of a substance is stable. In a phase diagram, pressure is plotted against temperature. The phase diagram below is for methane, CH₄. Note, the axes are not drawn to scale. In this diagram, the pressure is in atm and the temperature in Kelvin.

The solid lines divide up the solid, liquid, and gas regions. The phase in each of the regions is stable at those pressures and temperatures. The solid line between the solid and the gas regions represents both solid and gas at equilibrium and is called the sublimation (or deposition) curve. The solid line between the solid and liquid regions represents both solid and liquid at equilibrium, and is called the fusion (or solidification) curve, while the line between the liquid and gas regions indicates both liquid and gas in equilibrium and is called the vaporization (or condensation) curve. A change in temperature or pressure can result in a phase change. The solid lines are boundary lines between two phases. Anywhere on a boundary line indicates two phases in equilibrium. The triple point , is a point where all three phases are in equilibrium. For methane, the triple point is at 0.092 atm and 90.6K–solid, liquid and gas are at equilibrium at this point. The normal boiling point of methane is 111.6 K. Recall, the normal boiling point is at a pressure of 1 atm.

The critical point for methane is at 190.7 K under a pressure of 45.7 atm. At the critical point there is no difference between the liquid and gas phase. The densities of the liquid and the gas are equal at the critical point. Above the critical temperature, a gas cannot be liquified regardless of the pressure. Above the critical pressure, a liquid cannot be vaporized regardless of the temperature. Beyond the critical point is a supercritical fluid which is neither a liquid or a gas. The physical properties are intermediate between a liquid and a gas. A supercritical fluid can act as a solvent much like a liquid, and it is able to quickly extract a substance because the supercritical fluid flows like a gas. Carbon dioxide is used most often as a supercritical fluid because it only needs to reach a temperature of 31°C and a pressure of 73 atm. Ethyl acetate and dichloromethane are used to decaffeinate coffee. These chemicals are not completely removed from the coffee, and other compounds are also removed by these solvents which can affect the flavor of the coffee. Another method to decaffeinate coffee is supercritical carbon dioxide. It only removes the caffeine, does not leave behind any toxic compounds, and will not affect coffee flavor.

Notice, the solid-liquid boundary line for methane has a positive slope. This is because the density increases from liquid to solid. In the phase diagram below for water, the solid-liquid boundary line has a negative slope because solid water (ice) is less dense than liquid water.

Worksheet: Phase Diagrams

Exercises

Exercise 1. Answer the following questions pertaining to the phase diagram below for water.

Exercise 2. Draw and label a phase diagram for a substance with the following properties.