H2O phase diagram. Equilibrium Temperature (T) - Pressure (P) stability of three phases of H2O: water, ice, and vapor (steam). Notice that the pressure axis is logarithmic, whereas the temperature axis is linear. Only one H2O phase is stable at a time, except for the special conditions indicated by the black reaction curves for which two phases are stable. All three phases are stable together only at the special "triple point" (highlighted in red) where all three black lines intersect (T=273.16 K, P=611.657 kPa). There is also a red dot marking the critical point of H2O (T=647.096 K, P=22.064 MPa), above which there is no phase change between water and vapor (supercritical water).

To see the ice-water equilibrium point (273.15 K, 0°C) and the water-vapor equilibrium point (373.15 K, 100°C) at 1 atmosphere pressure (101.325 kPa), click the "Show 1 atm" button on the header bar. To see the linear-log T-P grid, click the "Show Grid" button on the header bar. CO2 has a similar stability diagram with a "triple point" and a critical point that can be seen here.

There are other phases of solid H2O that are stable at higher pressure. However, only one phase of H2O is stable at a time, except for special conditions along reaction curves. For more information, see the Water and Structure Page (now archived) by Martin Chaplin, which was the source of data for this figure. Some of the svg code for the diagram is from Wikimedia Commons User Cmglee.