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# Triple point

In physics, the triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium.

A typical phase diagram, with the triple point at the vertex where the three regions meet.

For example, the triple point temperature of mercury is -38.8344 °C at a pressure of 0.2 mPa.

The triple point of water is used to define the kelvin, the unit of thermodynamic temperature in the International System of Units. The number given for the temperature of the triple point of water is an exact definition rather than a measured quantity.

## Triple point of water

The single combination of pressure and temperature at which water, ice, and water vapour can coexist in a stable equilibrium occurs at exactly 273.16 kelvins (0.01 °C) and a pressure of 611.73 pascals (ca. 6 millibars). At that point, it is possible to change all of the substance to ice, water, or steam by making infinitesimally small changes in pressure and temperature. (Note that the pressure referred to here is the vapor pressure of the substance, not the total pressure of the entire system.)

At high temperatures, increasing pressure results in first liquid, and then solid water. At lower temperatures the liquid state ceases to appear with compression causing the state to pass directly from gas to solid. The triple point is the lowest temperature, for any value of the pressure, at which the liquid state is still passed.

At a constant pressure higher than the triple point, heating ice necessarily passes from ice to liquid then to steam. In pressures below the triple point, such as in outer space where the pressure is low, liquid water cannot exist; ice skips the liquid stage and becomes steam on heating, in a process known as sublimation.