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Phase Changes

Phase Diagram: Part 3

Phase Transitions caused by a Change in Pressure

Just as a change in temperature can drive a phase change, a change in pressure can produce the same effect.

In this exercise, the effect of a change in pressure at a constant temperature (isothermal conditions) is illustrated. Once again the sample is confined in a cylinder with a movable barrier. An increase in pressure will compress the sample, resulting in a lower volume and a higher density. Conversely, decreasing the pressure will result in an expansion of the sample, resulting in a higher volume and a lower density. During a phase change, the volume (and density) of the sample changes, but there is no change in the pressure.

The color of the sample indicates the sample's phase.

• Green indicates the solid phase
• Blue indicates the liquid phase
• Red indicates the gas phase

In this experiment the sample is held at an initial state (T, P) identifed as Point A. Throughout the experiment the temperature of the sample is held at a constant value of 220 K. Heat leaves or enters the sample from the surroundings as necessary to maintain this temperature. The controls adjust the volume of the sample. The position of the barrier (and thus the volume occupied by the substance) will change as the pressure on the system changes while maintaining a constant temperature.

Carefully vary the volume of the sample to move from Point A to Point B and observe the behavior of the sample in the cylinder. The current position in the phase diagram is marked by a purple dot. The longer the "Increase Volume" or "Decrease Volume" button is depressed (the mouse button held down), the faster the volume is increased or decreased. (The actual change in volume might be imperceptible. Compression of condensed phases requires a very high pressure, and even in a vacuum negligible expansion of a condensed phase occurs.)

1. What phase is stable at Point A?
2. What phase is stable at Point B?
3. At what pressure does condensation (vaporization) occur (at 220 K)?
4. At what pressure does freezing (melting) occur (at 220 K)?
5. Which phase has a higher density: the solid or the liquid? What feature of the phase diagram provides the answer to this question? How would the appearance of the phase diagram differ if the other phase (liquid or solid) were denser? Does this same reasoning apply to determining the relative densities of the solid and gas phases and of the liquid and gas phases?

 Phase Diagram Part 1 Part 2 Part 3 Part 4 Part 5