Critical Point of Benzene - 2 of 3
As the temperature rises, the liquid meniscus flattens. This flattening is due to the increased energy of the benzene molecules. The forces due to increased molecular movement are beginning to overcome the attractive molecular forces at the surface. At the edges of the vial, near the liquid surface, the benzene can be seen to be boiling. When the critical point is reached, the meniscus disappears and only diffraction irregularities are present due to temperature gradients. Here the forces due to molecular movement overcome the surface tension, and the separation between liquid and vapor disappears. Above the critical temperature, only the gaseous phase exists. A gas above the critical temperature cannot be condensed into a liquid, since the molecular attractive forces are dominated by the rapid molecular motion.
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