Triple Point: The temperature and pressure at which the fusion curve, the vaporization curve and the sublimation curve meet and all the three phases of a substance coexist is called the triple point of the substance. For example the triple point of water is represented by the temperature 273.16 K and pressure 6.11×10^–3 Pa.

Critical Point: The critical point, some times called critical state, occurs where the critical temperature and critical pressure of a substance meet. Above this point clear phase boundaries cease to exist. The critical point occurs at one specific temperature and pressure. For water the critical point is about 647 k and 22MPa. At the critical point, the properties of the gas and liquid phases merge together giving only one phase.

Key term

• Triple point is a point on a p - v diagram where the three phases of a substance can co exist.
• Critical point is a a point on a p - v diagram where the critical temperature and pressure of a substance meet.