Physics Photo of the Week
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.
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.
Photo of the
published weekly during the academic year on Fridays by the Warren
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