# The triple point of water

This is a most important point in the definition of the thermodynamic scale of temperature.

The triple point of any substance is the point where the solid, liquid and vapour states can exist in equilibrium.

The graph of pressure plotted against temperature the following diagram explains how it is defined.

If the graph is for water then the three lines shown represent:
(a) the boiling curve (AB) is the locus of points where water and its vapour can exist in equilibrium;
(b) the sublimation curve (AC) is the locus of points where ice and its vapour can exist together in equilibrium;
(c) the solidification curve (AD) is the locus of points where water and ice can exist together.

The triple point is at A where these three lines intersect and where water, water vapour and ice can exist in equilibrium. The temperature at which this occurs is defined as 273.16 K on the thermodynamic temperature scale. (The triple point is not exactly at 0 oC because under the pressure of its own vapour ice melts at about 0.0075 oC.)

© Keith Gibbs 2010