Triple Point

In-depth explanation of Triple Point

The triple point is the unique temperature and pressure at which the solid, liquid and gas phases of a substance coexist in equilibrium. It is a fixed point on a phase diagram where the three boundary curves meet. For water the triple point is at 0.01°C and 611.657 Pa (about 0.006 atm). Before 2019 the kelvin was defined by assigning the triple point of water exactly 273.16 K. Below the triple point pressure a solid sublimes rather than melting when heated. The triple point of each substance is characteristic of that substance.

Going deeper: Triple Point

Key ideas: Gibbs phase rule F = C - P + 2 gives zero degrees of freedom for a pure substance with three phases, so the triple point is fixed. Water: 273.16 K and 0.00604 atm. CO2: 216.6 K and 5.11 atm. Triple point cells are used to calibrate thermometers because they are reproducible.

Common mistakes and connections: Students confuse triple point with the normal melting point; water freezes at 0 C at 1 atm but 0.01 C at the triple point pressure. Another error is thinking a substance can only have one triple point; some have several involving different solid forms, such as sulfur. Links to phase diagrams, sublimation, critical point and temperature scales.

Visualization of Triple Point

Triple point of water is 273.16 K and 0.006 atm (611.7 Pa), where solid, liquid and vapour coexist; it defines the kelvin. The interactive lesson shows this as a live animation or chart.

Worked example and use case

Below the triple point pressure of water, ice sublimes rather than melting when it is heated.

Where Triple Point fits in States of Matter & Solutions

Triple Point is topic 51 of 109 in States of Matter & Solutions.

Related topics

• SublimationBeginner
• DepositionBeginner
• Phase DiagramIntermediate
• Critical PointIntermediate
• Supercritical FluidsAdvanced
• Phase Diagram of WaterIntermediate

Browse all 109 topics in States of Matter & Solutions