A "phase diagram" is used by chemists to display graphically the different solid, liquid, and gas phases--as well as mixtures of these--that exist in a pure compound or combination of compounds. Depending on the situation, the diagram may show the variation of phases with temperature, pressure, percentage of one component of a mixture, or other variables.
Figure 1 shows a temperature-pressure phase diagram for Substance 1, a pure compound. Figure 2 is a similar diagram for Substance 2. Figure 3 shows the temperature behavior of the phases in a mixture of aluminum which contains a small, variable amount of silicon. Note that this diagram refers to two different solids, labeled "solid 1" and "solid 2." Each of these solids contains a mixture of aluminum and silicon, but they differ in percent composition and in crystal structure.