CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 181
Note:The wordotheris used here to emphasize that the conditions of phase equilibrium
(e.g. (7.2)) are already comprised in the formula.
Example
Give the number of degrees of freedom at boiling: a) for a pure substance, b) for a mixture.
Solution
We find the number of degrees of freedom using equation (7.7).
Ad a) In a pure substance there are two phases at boiling andC= 0:
v= 1−2 + 2−0 = 1.
The system has one degree of freedom, its state being unambiguously determined either by the
boiling temperature or by the saturated vapour pressure.
Ad b) A binary mixture has two degrees of freedom at boiling
v= 2−2 + 2−0 = 2.
and the state of the system is unambiguously determined by either of the following pairs of data:
T, p,orT, x 1 , orT, y 1 , orp, x 1 , orp, y 1 , orx 1 , y 1 , wherex 1 is the molar fraction of component
1 in liquid, andy 1 is the molar fraction of component 1 in vapour.
