Physics Photo of the Week

February 1, 2008

Critical Point in Gas-Liquid -
Discussion by Chelsea Maier

The critical point for a substance is the temperature above which the gas phase of the substance can no longer be condensed into liquid no matter what pressure is applied.  The kinetic energy between the molecules of the substance is so great that being in a liquid phase is impossible, but the pressure is so high that the molecules exist in a super dense gas.  Rather, the gas and the liquid phase have the same density and they are indistinguishable from one another.  In the picture above (left) a steel closed container containing Freon115 (1-ChloropentaflouroEthane :  ClF2 C - C-F3) at a pressure of 4 atmospheres is heated above the critical point of Freon 115 in a water bath.  Inside the glass window, the interface between gas and liquid is gone; the density of the gas molecules of Freon115 are the same as the liquid molecules of Freon 115.  There is no distinguishable feature in the container.
Large animated file - please wait

When the container was cooled below the
critical point of the Freon 115, the super dense gas phase condensed in random places within the container, and the condensation fell out of gas form.  This phenomenon is shown in the top right picture.  Even though the Freon 115 is still warm, it is not above is critical point and therefore exist as a liquid. 
The animation here shows the process of the gas condensing out of the critical state into clouds, then as a pool of liquid.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to

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