Liquids, Solids, and Phase Changes
Interactive Student Tutorial

 10.11 Phase Diagrams

A phase diagram is 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 point represents a combination of temperature and pressure at which all three phases exist in equilibrium.
  • The normal melting point is the temperature at which the solid/liquid boundary line reaches 1 atm.
  • The normal boiling point is the temperature at which the liquid/gas boundary line reaches 1 atm.
  • 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.