Water can boil and freeze at the same time.

Boiling depends on pressure as well as temperature. A liquid boils when its vapor pressure matches the pressure surrounding it, allowing vapor bubbles to grow within the liquid. Lower the surrounding pressure sufficiently and water can boil without becoming hot enough to burn you, unlike a saucepan boiling at ordinary atmospheric pressure.

Boiling also takes energy. Molecules leaving the liquid carry energy away, and the remaining water cools unless heat arrives quickly enough from its surroundings. In an evacuated container, this cooling can bring some water to its freezing point while other portions continue producing vapor. Heat flow and pumping determine what happens next.

The triple point describes a particular equilibrium at which three phases coexist. It is not simply another name for every vacuum experiment that shows ice and bubbles. A dramatic demonstration can involve changing pressure, uneven temperatures and continuing evaporation, whereas an equilibrium triple point has well-defined conditions throughout the sample.

Scientists map these possibilities with a phase diagram, a graph showing which state is stable at different temperatures and pressures. The boundaries between solid, liquid and vapor meet at the triple point. For ordinary water of specified isotopic composition, its temperature is approximately 273.16 kelvin, equivalent to 0.01 °C.

The corresponding pressure is approximately 611 pascals, compared with standard atmospheric pressure of 101,325 pascals. That is about six thousandths of an atmosphere. Ordinary room conditions therefore do not reproduce the experiment, even if ice cubes, liquid water and humid air happen to be present in the same kitchen.

Water triple-point cells became important tools in precision thermometry. A sealed cell containing carefully prepared water can provide a reproducible temperature reference when ice, water and vapor are established correctly. Purity, isotopic composition and measurement technique matter because small differences become significant when a laboratory is measuring tiny temperature changes.

Until 2019, the kelvin was defined through the triple-point temperature of water. The international system now defines it using an exact value of the Boltzmann constant. The change did not make triple-point cells obsolete: they remain useful practical references, but water no longer supplies the fundamental definition of the temperature unit.

An ice cube melting in a glass is not evidence that the whole glass is at the triple point. Air above the water contains several gases, and water vapor can be present over a wide range of temperatures. Coexistence somewhere in a room is different from three pure phases sharing equilibrium conditions.

Below the triple-point pressure, liquid water is not the stable equilibrium phase. Ice can change directly into vapor, a process called sublimation. Freeze-drying uses low pressures and controlled heating to remove water from frozen material. The goal is controlled sublimation rather than keeping an ordinary liquid boiling indefinitely.

A phase diagram is a map of possibilities, not a timetable for what every sample must do. Water can remain temporarily outside its most stable state, so an experiment's starting conditions and heat supply matter alongside its position on that map.