Definition - What does Phase Diagram mean?
A phase diagram is a graphical representation showing different phases of a substance or a mixture of substances that coexist in a thermodynamic equilibrium and undergo phase changes at different operating conditions such as temperature, pressure or volume.
There are three phases in which a substance can exist: solid, liquid or gas. On a typical phase diagram, pressure is represented on the y-axis and temperature is represented on the x-axis.
Corrosionpedia explains Phase Diagram
A phase diagram depicts the phase change of the substance at, for example, a certain pressure and temperature. A phase diagram may represent a PT curve (pressure, temperature curve). The PT curve is formed by taking readings at various pressure and temperature conditions.
A PT curve tells us that at any point in the areas separated by this curve, only one phase of a substance exists (i.e., either the substance will exist in a solid, liquid or gaseous phase) and changes in the pressure and temperature on the points of the curve will not change the phase of the substance. However, this curve also represents that at any point on the curve at a different pressure and temperature condition, the two phases of a single substance remain in thermodynamic equilibrium with each other.