Phase Changes Diagram

By: Katie Lane

What is It?

It is a diagram, or map, that shows how changes in pressure and temperature determine the phases of a substance, such as water. This determines if the substance is a solid, liquid or a gas.

What Is Shown?

On a phase diagram, the conditions of pressure and temperature at which two phases exist in equilibrium are indicated by a line separating the phases.

A Phase Diagram

Explaining the Diagram

In each of the colored regions, the substance is in a single phase. The curving lines that separate the phases are the equilibrium conditions. The lines also illustrate how pressure of the substance varies with its temperature. The point on the diagram where all three curves meet is called the triple point. The triple point describes the only set of conditions where all three phases can exist in equilibrium with each other.


The graph can help you determine the relationships between pressure and temperature. If there is a decrease in pressure, this will lower the boiling point. An increase in temperature will raise the boiling point and lower the melting point. The graph also helps you to see where the physical states of the substance are at different conditions.