Lake Ice and 
Snow Science 

BASIC CONCEPTS: H₂O Phase Diagram

A phase diagram is a graphical way to depict the effects of pressure and temperature on the phase of a substance. This water phase diagram shows whether a substance exists as a vapor, liquid, or solid at a given temperature and pressure.

Source: http://wine1.sb.fsu.edu/chm1045/notes/Forces/Phase/Forces06.htm

The curves indicate the conditions of temperature and pressure under which equilibrium between different phases of a substance can exist. The triple point occurs where the three lines intersect in a phase diagram and indicates the pressure and temperature where solid, liquid, and vapor all exist in equlibrium.

The critical point is the thermodynamic state in which liquid and gas phases of a substance coexist in equilibrium at the highest possible temperature. At higher temperatures than the critical no liquid phase can exist.

For water the critical point is:

e_s = 2.21 x 105 mb

T = 647°K

α = 3.1 gm cm^-3

where e_s is the saturation vapor pressure of the water vapor, T is the Kelvin temperature, and α the specific volume. (The specific volume is the volume per unit mass of a substance, and hence the reciprocal of density.)

A torr is a non-SI unit of pressure and is the pressure exerted by 1mm of mercury (Hg). A standard atmosphere of pressure equals 760 torr or 1 torr equals 0.001316 atmospheres.

QUESTION: At what temperature do ice, liquid water and water vapor exist in equilibrium?