Triple point

Below this, in the vacuum of outer space, solid ice sublimates, transitioning directly into water vapor when heated at a constant pressure.

Conversely, above the triple point, solid ice first melts into liquid water upon heating at a constant pressure, then evaporates or boils to form vapor at a higher temperature.

The melting point of ordinary ice decreases with pressure, as shown by the phase diagram's dashed green line.

Historically, during the Mariner 9 mission to Mars, the triple point pressure of water was used to define "sea level".

For those high-pressure forms of ice which can exist in equilibrium with liquid, the diagram shows that melting points increase with pressure.

For exacting work, triple-point cells are typically filled with a highly pure chemical substance such as hydrogen, argon, mercury, or water (depending on the desired temperature).

Triple-point cells are so effective at achieving highly precise, reproducible temperatures, that an international calibration standard for thermometers called ITS–90 relies upon triple-point cells of hydrogen, neon, oxygen, argon, mercury, and water for delineating six of its defined temperature points.

A typical phase diagram . The solid green line applies to most substances; the dashed green line gives the anomalous behavior of water
Water boiling at 0°C using a vacuum pump.
Phase diagram of water including high-pressure forms ice II, ice III, etc. The pressure axis is logarithmic. For detailed descriptions of these phases, see Ice .