Liquids, Solids, and Phase Changes
Interactive Student Tutorial
10.11 Phase Diagrams
A phase diagramis a graphical way to illustrate the phase properties of a substance under different conditions of temperature and pressure.
Phase Diagram of Water
Note the axes on a phase diagram: pressure is plotted as a function of temperature.
Points on the line segments represent pressure/temperature combinations at which two phases are in equilibrium. Increasing either temperature or pressure from a particular point results in a phase transition.
The triple pointrepresents a combination of temperature and pressure at which all three phases exist in equilibrium.
The critical point is the point at the end of the liquid/gas boundary line. The critical temperature represents the temperature at which a gas cannot be liquefied, and the critical pressure is the pressure beyond which a liquid cannot be vaporized at any temperature. The two phases blend to form a
supercritical fluid that is neither liquid nor gas beyond the critical point.
A phase diagram is a useful way to display in graphical form a tremendous amount of information about the physical states of a substance.