Phase Diagram Worksheet – Triple Point and Critical Point

All 11 parts on the diagram

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Start here: 5 main parts 5

Solid The region of temperatures and pressures where the substance is a solid.
Liquid The liquid region: here the substance is a liquid.
Gas The gas region, or vapor: here the substance is a gas.
Triple point The one temperature and pressure where solid, liquid and gas exist together.
Critical point The critical point: the end of the vaporization curve. Above it the substance is a supercritical fluid.

The axes: 2 2

Pressure Pressure, rising up the side axis.
Temperature Temperature, rising along the bottom axis.

The regions: 4 1

Supercritical fluid Above the critical temperature and pressure. Here liquid and gas can no longer be told apart.

Curves and points: 5 3

Melting curve Along it solid and liquid are in equilibrium. Also called the freezing curve. It gives the melting point at each pressure.
Vaporization curve The vaporization curve: along it liquid and gas are in equilibrium. It gives the boiling point at each pressure.
Sublimation curve Along it solid and gas are in equilibrium. Here a solid turns straight to gas.

How this list was compiled: Read 2026-09-27. (1) OpenStax, Chemistry 2e, section 10.4 Phase Diagrams, https://openstax.org/books/chemistry-2e/pages/10-4-phase-diagrams : a phase diagram combines plots of pressure against temperature for the liquid-gas, solid-liquid and solid-gas equilibria; it shows the stable state at each temperature and pressure; the liquid-vapor curve gives the boiling point at each pressure and ends at the critical point; the solid-vapor curve gives the sublimation points; the solid-liquid curve gives the melting and freezing points and for most substances has a positive slope, water being the notable exception; the triple point, where all three meet; below the triple-point pressure there is no liquid; above the critical point the substance is a supercritical fluid. (2) Wikipedia, Phase diagram, https://en.wikipedia.org/wiki/Phase_diagram : the same features and names, with pressure on the vertical axis and temperature on the horizontal. Licence: own work

The drawing: Drawn for this site by script, 2026-09-27. Own work, nothing traced. Licence: own work

How it works

This phase diagram worksheet is a blank pressure-temperature diagram for a typical substance. Label it on screen, quiz yourself, or print it. Start with the 5 main parts.

Read the full guide

Reading it

Temperature runs along the bottom and pressure up the side. Each region shows the state the substance is in there: solid, liquid or gas. The curves between them show where two states are in balance.

The special points

The three curves meet at the triple point, where solid, liquid and gas exist together. The vaporization curve stops at the critical point; beyond it the substance is a supercritical fluid.

Water is the odd one out: its melting curve leans left, because ice melts at a lower temperature when the pressure rises.

Questions and answers

What is the triple point on a phase diagram?

The single temperature and pressure where the solid, liquid and gas states all exist together in balance. It is where the three curves meet.

What is the critical point?

The end of the vaporization curve. Above the critical temperature and pressure, liquid and gas merge into one state, a supercritical fluid.

What does the sublimation curve show?

The temperatures and pressures where a solid turns straight into a gas, or a gas straight into a solid. It lies below the triple point, where no liquid can form.