Phase Changes

Phase Diagram: Part 3 EXPERIMENT F

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). Once again, the sample is confined in a cylinder with a movable barrier. Attempts to increase the volume of the sample produce a decrease in temperature, unless a phase change occurs, in which case the sample changes to a higher volume phase in order to maintain the pressure. Conversely during a phase change a decrease in volume converts the sample to a higher density phase in order to maintain the pressure. In the absence of a phase change and increase or decrease in volume produces a corresponding decrease or increase in pressure.

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

In this experiment the sample is held at an initial state (T, P) identifed as Point A. Throughout the experiment the pressure on the barrier is held at a constant value of 3.00 atm (ln (P/atm) = 1.099). The controls adjust the temperature of the system. The position of the barrier (and thus the volume occupied by the substance) will change in response to the change in temperature in order to maintain a constant pressure (isobaric conditions).

The solid phase is represented by a green color, the liquid phase by a blue color, and the gas phase by a red color.

Carefully vary the pressure 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)?

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

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