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Phase Changes

Phase Diagrams: Part 2

Phase Transitions

A phase diagram may be employed to determine whether a particular change in conditions produces a phase change. Information in a phase diagram also permits the determining of boiling points, melting points, and sublimation points.

A phase diagram for a substance is shown below. At the right is a cylinder containing a pure sample of the substance. The sample is confined in the cylinder by a barrier, and the pressure on the barrier may be controlled. The temperature of the sample may also be controlled.

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).

Carefully vary the temperature 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 "Heat Sample" or "Cool Sample" button is depressed (the mouse button held down), the faster the sample is heated or cooled.

1. What phase is stable at Point A?
2. What phase is stable at Point B?
3. Is there any temperature (at 3.00 atm pressure) at which a phase transition occurs? If so, what is the transition (fusion, sublimation, or vaporization)?
4. What is the temperature at which this phase transition occurs?
5. What is the normal temperature of sublimation? (Recall that normal refers to P = 1 atm.)











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