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After this lesson, you should be able to...

Sketch a phase diagram for a substance, including the triple point and thecritical point.
Use a phase diagram to determine the phases of matter that will exist for a substance at specific temperatures/pressures.
Use a phase diagram to determine the changes that will occur to a pure substance as temperature or pressure (or both) are changed.
Explain how the phase diagram for water differs from most substances.
Relate the heating curve to the phase diagram of a pure substance.

Phase diagrams are a graph of pressure plotted against temperature. Pressure is defined as a force per area due to molecular collisions. Atmospheric pressure is the force that gas particles in the atmosphere exert. The melting and boiling points vary with pressure. The "normal" melting or boiling point is at a pressure of 1 atm. As pressure changes, the temperature of the phase change will also change. Melting is only slightly influenced by changes in pressure and may increase or decrease depending on the substance. However, the boiling point is significantly impacted by pressure. The substance vaporizing must exert a pressure equal to the atmospheric pressure in order to boil. Therefore, at higher pressures (more force), the boiling point will increase.

Phase diagrams can give a few useful pieces of information:

Phase(s) present under specific conditions (temperature and pressure)
Normal boiling and melting points (when pressure is 1 atm)
The boiling or melting points at any pressure
Predict changes in state that may occur with changing temperature and/or pressure

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