Learning Outcomes

At the end of this section, you should be able to:

  • Interpret the phase diagram for one component system

A phase diagram is a graph of pressure and temperature that shows the conditions under which a substance’s phases exist. It shows how the state of a system changes with changing temperature or pressure.

The Features of a Phase Diagram

Let’s use the phase diagram of water to examine the major features of a phase diagram.


  • The y-axis displays the pressure in atm
  • the x-axis displays the temperature in degrees Celsius
  • There are three main regions: Solid, liquid, and gas
  • there are three main lines: AB, AC, and AD
  • there are two major points:
    1. Triple point
    2. Critical point


A region represents the state at which water is stable at a given temperature and pressure.

For example, water is stable as a solid (ice) in the solid region. Notice that the pressure at 1.00 atm and temperature at 0.00 ℃ fall within the solid region of the diagram.

Phase preference:

  • Low temperature and high pressure favour a solid state.
  • High temperature and low pressure favour a gas state.
  • Intermediate conditions favour a liquid state.

Phase Conversion:

  • A sample of matter will convert to its corresponding state as indicated by its phase diagram when:
    • The conditions (temperature and pressure) are imposed
    • The sample is not already in the favoured state
  • Example: Steam cooled to room temperature (20°C) at 1 atm will condense to liquid water.


Each line (or curve) in the phase diagram represents a set of temperatures and pressures at which the substance is in equilibrium between the two states on either side of the line.

The curved line AB separates the liquid from the gas in the water phase diagram. This line is called the vaporisation curve or vapour pressure curve for water. Along this line, the liquid and gas states of water are equally stable and in equilibrium.

For example, at 100 °C and 1.00 atm pressure, water and water vapour are in equilibrium-they are equally stable and will coexist. The other two major lines in a phase diagram are the sublimation curve (separating the solid and the gas) and the fusion curve (separating the solid and the liquid).

The Triple Point

  • The triple point on a phase diagram signifies the conditions where three phases coexist in equilibrium.
  • This point is at 0.01 °C and 0.0060 atm for water.
  • Only at this temperature and pressure do the solid, liquid, and gas phases of water coexist in equilibrium.

The Critical Point

  • This point shows the specific temperature and pressure where a liquid and gas become indistinguishable, forming a supercritical fluid.
  • Above this critical point, the substance exists only as a supercritical fluid. At this point, the distinction between liquid and gas disappears, resulting in a fluid with unique properties of both.