CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 174
chosen pressurep. Thedew pressureis a pressure at which a vapour of a given composition
is in equilibrium with its liquid at a given temperature.7.1.7 Saturated vapour pressure
The saturated vapour pressure is a pressure in a one-component system at which the gaseous
phase is in equilibrium with the liquid or solid phase at a given temperature. The saturated
vapour pressure is the highest pressure at which a substance can be in the equilibrium gaseous
state at a given temperature. It is also the lowest pressure at which a substance can exist in
the liquid or solid equilibrium state at a given temperature.
S Symbols:The saturated vapour pressure of componentiwill be denotedpsi.
Note:The term saturated vapour pressure is used for pure substances only. The substances
which have a higher pressure of saturated vapour at a given temperature are calledmore
volatile.Example
In all substances, the saturated vapour pressure at the normal boiling point equals 101 325 Pa.Note:It often happens that there is a gas (usually air) of pressurepover a liquid. If at
a given temperaturepsi< p, we say that the liquidvaporizes, ifpsi=p, we say that the
liquidboils.7.1.8 Melting point
Themelting pointis determined by the melting temperature, melting pressure and the com-
position of the solid phase. Themelting temperatureis a temperature at which the solid
phase of a given composition is in phase equilibrium with its liquid at a chosen pressurep. The
melting pressureis a pressure at which the solid phase of a given composition is in phase
equilibrium with its liquid at a chosen temperature.