# Fluids in the Critical Region

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On the left is a phase diagram of the critical region of a representative fluid. Within the shaded area, the fluid separates into two phases: liquid and gas. The critical isotherm is also shown, labeled by the critical temperature . At the critical point, this isotherm is tangent to the two-phase region. A gas can be liquefied only after it is cooled below its critical temperature.

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Contributed by: S. M. Blinder (March 2011)

Open content licensed under CC BY-NC-SA

## Snapshots

## Details

Snapshot 1: This state would be identified as a liquid at a given pressure.

Snapshot 2: At a much higher temperature, the system would be a gas at the same pressure.

Snapshot 3: Going from state 1 to state 2 at constant pressure, the system would go through the two-phase region.

Snapshot 4: An alternative way to go from state 1 to state 2 would be to follow a path around the critical point. The same final state would then be attained continuously, *without* any phase separation. This is known as the principle of continuity of states.

Snapshot 5: As the critical point is approached, the densities of the two phases approach one another.

Snapshot 6: The densities become equal at the critical point.

Reference: S. M Blinder, *Advanced Physical Chemistry; A Survey of Modern Theoretical Principles*, New York: Macmillan, 1969 p. 134.

## Permanent Citation

"Fluids in the Critical Region"

http://demonstrations.wolfram.com/FluidsInTheCriticalRegion/

Wolfram Demonstrations Project

Published: March 7 2011