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Critical Point of Benzene

A sealed tube containing benzene liquid and vapor is heated. As the critical temperature is approached, the meniscus flattens. At the critical temperature the meniscus disappears, and separate liquid and vapor phases can no longer be detected. When the apparatus is cooled below the critical temperature of benzene, the meniscus reappears.

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phase changes, states of matter, physical properties, gases and liquids, critical point, solids and liquids, bonding


_Play movie (15 seconds, 0.9 MB)

A sealed vial containing only benzene liquid and vapor is placed in a Thiele tube which is then filled with silicone fluid. Heat is applied with a Bunsen burner.

Temperature increases
_Play movie (1 minute 6 seconds, 3.6 MB)

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.

Temperature decreases
_Play movie (38 seconds, 2.1 MB)

As the temperature decreases, the meniscus reappears at the critical temperature. The molecules in the gas move more slowly, allowing the attractive forces between the molecules to become dominant. At this point separate liquid and vapor phases form. Note that the thermometer is present only to indicate increasing and decreasing temperature and an approximate range for the critical temperature.


Benzene Meniscus at Low Temperature

Compare this with the meniscus at high temperature.

The meniscus simply disappears as the benzene is heated above its critical temperature, and the meniscus simply reappears as the benzene cools to below its critical temperature. The benzene does not boil or condense (the meniscus does not fall to the bottom or rise from the bottom of the tube). The critical temperature is the temperature above which the distinction between liquid and gas no longer applies, so as the benzene is heated above its critical temperature the difference between gas and liquid disappears and the benzene becomes one fluid (usually called gas), and conversely as the benzene is cooled.

Additional still images for this topic

Demonstration Notes: Warnings, Safety Information, etc.

Exam and Quiz Questions

1. Why does the shape of the meniscus change as the benzene is heated?

2. Above the critical point, what phases exist?

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