Phase diagrams
From APChempedia, the free AP prepbook
Phase diagrams express the physical state of a substance in terms of its temperature and pressure exerted. The triple point corresponds to the temperature and pressure where all three phases of a substance exist. On the other hand, the critical point refers to the temperature above which a gas cannot be liquefied no matter how much pressure is applied to it.
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Phases at the Molecular Level
Animation of molecular movement of a unknown liquid:
When a substance is in a liquid phase, its atoms have fluid movement and take the shape of the container in which they are stored.
Animation of molecular movement of a unknown gas:
Since the substance is in the gas phase, its atoms move randomly. They also occupy a larger volume and have a lower density.
Phase Diagrams Exposed
Phase diagrams are graphs that depict the phase of a substance in terms of pressure and temperature. By knowing a substance’s temperature and pressure, a phase diagram will determine the phase or current state of matter of the substance.
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The phase diagram illustrates the phases of a substance in terms of its temperature and pressure. The branching lines in the actual diagram serve as the boundaries for the three phases. Several important points of a phase diagram include the triple point (A) and critical point (B). The triple point (A) refers to the temperature and pressure at which all three phases of a substance can exist.
The critical point (B) is determined by the critical temperature and critical pressure. The critical temperature is the temperature above which a gas cannot be liquefied no matter what pressure is applied to it. The critical pressure is the pressure required to produce a liquid.Given a phase diagram, determine the phase at a given temperature and pressure
To determine the phase when given a phase diagram, first determine the location of the given temperature and pressure on the axis. Now, find where the two values intersect on the graph itself. The location of the point of intersection determines the phase of the substance.
Example:
Given: the phase diagram for water (see above), water's temperature is 100 degrees Celsius with a pressure of 1000 torr.
First, convert the 1000 torr to atm because the graph has pressure given in atmospheres. Then, locate the 100 degree mark on the temperature axis. Since 1000 torr is equal to 1.316 atms, the two points intersect where water is a liquid.
Practice Phase Diagram Problems
The phase diagram for a pure substance is given below:
Timoslav likes to store unknown pure substances in his closet. One day, he finds a contained marked "unknown substance stored at 50 atm and 50 K." He also found a phase diagram of the substance on the bottom of the container. What would happen to Timoslav if he opened the container ?
A) The solid in the container sublimes.
B) The liquid in the container vaporizes.
C) The vapor in the container escapes.
D) The solid in the container remains unchanged.
The correct answer is D. At 50 atm and 50 K, Timoslav's unknown substance is a solid.
For further practice problems, we have integrated phases and heating/cooling curve problems located under heating/cooling curves.
