The diagram you mostly find associated with different phases of a substance is the so-called “phase diagram”. This diagram shows the regions of stability of different phases as a function of temperature and pressure. The interfaces between these regions will be the phase transition lines.
The phase diagram for CO2 is shown below.
Things to notice on the diagram.
1. Given a pressure and a temperature you can find the stable phase (gas, solid, or liquid)
2. As pressure and temperature are varied CO2 behaves as expected:
The key to a phase diagram is the phase transition lines. For example, in the lower left portion of the diagram is the sublimation line that divides the solid and the gas. At the interface between the solid and the liquid is the melting line. When interpreting a phase diagram, it is critical to note that the pressure that is important for the gas is the partial pressure of the substance you are looking at. Thus, it is best to think about the pressure in this graph as the pressure of CO2. This is very important as it means that the sublimation line and the vaporization line actually are the vapor pressure of the solid and the liquid. Later we will examine the mathematical relation that determines the temperature dependence of the vapor pressure.
Below is another phase diagram, for water. Note how the solid/liquid line (ice/water line) has a negative slope. This means that you can melt ice by applying a higher pressure. Most substances (like CO2 above) have a positive solid/liquid line.
Interpreting Phase Diagrams.
In the above diagram for water what is the most stable phase at a temperature of 250K and a pressure of 100 bar?
© 2013 mccord/vandenbout/labrake