What is the temperature of the triple point of water in Kelvin?

The temperature of the gas–liquid–solid triple point of water is 273.16 K.

At this special temperature and a pressure of about 611.657 pascals, ordinary ice, liquid water and water vapour can coexist in stable equilibrium. The triple point is therefore not simply water’s everyday freezing or boiling temperature; both temperature and pressure must be specified. At normal atmospheric pressure, water freezes near 273.15 K, which explains the tempting but incorrect alternative.

Before 2019, the triple point of water defined the kelvin so that it was exactly 273.16 K. The International System of Units now defines the kelvin using the Boltzmann constant, making the water triple point an experimentally measured reference rather than the unit’s definition.

Water also has other triple points involving high-pressure forms of ice, but ordinary quiz questions mean the gas–liquid–solid triple point. In Celsius, its temperature is approximately 0.01°C, slightly above the conventional freezing point at one atmosphere.