Phase diagram

Simplified temperature/pressure phase change diagram for water.

A phase diagram is a diagram that describes a physical system (an element or a mixture). It shows under what conditions the system is in what state. Often, it is a two dimensional diagram that maps parameters (such as temperature and pressure). For each point, it will assign a specific phase (either solid, liquid, or gas). Such diagrams are widely used in material science, physics and chemistry.

Metastable states are usually not shown in a phase diagram. The lines in such a diagram show phase changes, for example from solid to liquid. On the line, the substance can be in either phase.

Phase diagrams describe a system in a stable state, in equilibrium.