CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 199
Figure7.6 presents a system with positive deviations from Raoult’s law. The lower
curve is thecurve of dew points[see7.1.6], the upper curve is thecurve ofboiling
points[see7.1.4]. In the region between these two curves, the system is heterogeneous.
In the two-phase region, the composition of both phases is unambiguously given for a
fixed temperature and a chosen pressure, as can be seen in the diagram and as follows
from the Gibbs phase rule [see7.3]. Similarly, for a chosen temperature and composition
of one phase, the composition of the second phase and the pressure are fixed.
- Isobaric diagram
Figure7.7. shows the curve of dew temperatures (the upper curve) and the curve of
boiling temperatures (the lower curve). In the two-phase region there is one degree of
freedom at constant pressure, at point A there are zero degrees of freedom (another fixed
condition,y 1 =x 1 , occurs—see (7.7)).
- y–xdiagram
For every isobaric or isothermal diagram, we may draw a corresponding diagram of the
vapour phase—liquid phase composition, the so-calledy–xdiagram [see Figure7.8].
7.6.6 Azeotropic point
- If for coexisting phases in ak−component system
yi=xi, i= 1. 2 ,... , k , (7.26)
[see point A in Figure 7.7], we speak about anazeotropic point or, shortly, an
azeotrope.
- At the azeotropic point it also applies that
(
∂p
∂xi
)
az
= 0,
(
∂p
∂yi
)
az
= 0, [T], i= 1. 2 ,... , k , (7.27)
or (
∂T
∂xi
)
az
= 0,
(
∂T
∂yi
)
az
= 0, [p], i= 1. 2 ,... , k. (7.28)
- For an ideally behaving vapour phase, combining (7.24) and (7.26) gives
γi
γj
=
psj
psi
. (7.29)
