Water can boil and freeze at the same time
At one exact temperature and pressure, water exists as ice, liquid, and vapour all at once — a point so reliable it defined the kelvin.
Get the temperature and pressure exactly right and water does something strange: it freezes, melts, and boils simultaneously, all three phases coexisting in equilibrium. This is the triple point.
For water it sits at 273.16 K (0.01 °C) and a pressure of about 611.7 pascals — roughly 1/166th of sea-level air pressure. Nudge either value and the balance tips toward ice, liquid, or vapour.
Because the triple point is so precise and reproducible — far more reliable than a freezing point, which shifts with pressure and impurities — scientists made it a cornerstone of measurement. From 1954 until 2019, the kelvin was defined as exactly 1/273.16 of the triple-point temperature of water.
The kelvin is now fixed by the Boltzmann constant, but sealed triple-point cells still calibrate the world’s most precise thermometers.
Sources & references
2 referencesWell-established. Corroborated by 2 independent sources.
