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.)
