We often learn that water is solid, liquid or gaseous. The word or can make you forget a possibility: under certain well-defined conditions of pressure and temperature, the three phases can exist together at equilibrium.
This is called the triple point. A cell presented by NIST contains water of specified isotopic composition in a closed glass enclosure. Ice, liquid and steam provide a particularly useful reference state for temperature measurements.
The phrase “water boils and freezes at the same time” attracts attention, but sometimes suggests a restless drop of contradictory behavior. On the contrary, scientific interest comes from the balance between phases. The phenomenon recalls that a temperature alone does not tell everything: the pressure and the composition of the system also count to determine the state of the matter.
What we know
Coexistence with the equilibrium of the three phases; no arbitrary bubbling and freezing under ordinary conditions.
To go to the source
References consulted on September 21, 2026. Linked pages may be in another language.
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