CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 175
7.1.9 Normal melting point.
Thenormal melting pointis determined by the normal melting temperature, normal pressure
p=101 325 Pa and the composition of the solid phase. Thenormal melting temperatureis
a temperature at which the solid phase of a given composition is in phase equilibrium with its
liquid at the normal pressure.
Example
The normal melting temperature of water is 273.15 K.7.1.10 Freezing point
The freezing pointis determined by the freezing temperature, freezing pressure and the
composition of the liquid phase. In a pure substance, the freezing point is identical with the
melting point. Thefreezing temperatureis a temperature at which a liquid of a given
composition is in phase equilibrium with its solid phase at a chosen pressurep. Thefreezing
pressureis a pressure at which a liquid of a given composition is in phase equilibrium with its
solid phase at a chosen temperature.
7.1.11 Triple point
The triple point is a particular temperature and pressure in a one-component system at which
three different phases can coexist in equilibrium. These three phases are normally solid, liquid
and gaseous. However, there are also triple points at which two solid and one liquid, or two
solid and one gaseous, or three solid phases are in equilibrium.
Note: The term triple point is used only for pure substances. The system at the triple
point has no degree of freedom [see7.3.4].Example
Water has one of its triple points at 0.01◦C and 611 Pa, the temperature and pressure at which
ice, liquid water and water vapour are in equilibrium. Besides this one, water has at least six
more triple points.