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Triple and critical points
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Special conditions for phase coexistence and fluid merging.
At the triple point three phases coexist (water: 273.16 K, 611 Pa). Above the critical point (water: 374 °C, 22 MPa) liquid and gas become one supercritical fluid.
Supercritical CO₂ decaffeinates coffee.
Defines the Kelvin scale.
In more detail
The triple point is the unique temperature and pressure at which solid, liquid and gas coexist. The critical point is the end of the liquid-gas boundary, above which the distinction disappears.
Real-world uses
- The triple point of water defines the kelvin scale.
- Supercritical fluids are used in decaffeination and extraction processes.
- Water's triple point, where solid, liquid and vapour coexist, was the reference for defining the kelvin until 2019, and it remains a standard calibration point.
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Frequently asked questions
What is the triple point of water?
About 273.16 K and 611 Pa.
What happens above the critical point?
The substance becomes a supercritical fluid, with properties between a gas and a liquid.
What is the critical point?
The temperature and pressure above which liquid and gas cannot be distinguished, so the substance is a supercritical fluid.
Related topics in Heat & Thermodynamics
Latent heat · Calorimetry · Phase diagrams · Conduction · Thermal resistance · Convection