Phase diagram

Phase diagrams show the variation in phase of a substance with pressure and temperature,

 

  • the solid being present at low temperatures and higher pressure
  • the liquid at intermediate temperatures and pressures
  • and the gas at higher temperatures and lower pressures.


Pressures and temperatures at which two phases are at equilibrium are represented by lines.
Hence

  • the liquid-gas line shows the change in boiling point with pressure.
  • the solid-liquid line shows the change in melting point with pressure.


The critical temperature (the end of the liquid-gas line) is the highest temperature at which the liquid and gas can coexist.

Above the critical temperature the gas can't be liquefied by pressure alone.


The triple point is the point at which the liquid-solid, solid-gas and liquid gas lines intersect.

At the triple point the three phases are in equilibrium.
Dr Sheila Woodgate
© The University of Auckland 2002