Thermodynamics and Chemistry

CHAPTER 8 PHASE TRANSITIONS AND EQUILIBRIA OF PURE SUBSTANCES

8.2 PHASEDIAGRAMS OFPURESUBSTANCES 209

bc

(^00100200300400500)
50
100
150
200
250
300
.V=n/=cm^3 mol^1
p=
bar
350 ÆC
300 ÆC
250 ÆC
200 ÆC
400
ÆC
500
ÆC
600
ÆC
700
ÆC
800
ÆC
900
ÆC
1000
ÆC
Figure 8.10 Isotherms for the fluid phases of H 2 O.a The open circle indicates the
critical point, the dashed curve is the critical isotherm at373:95C, and the dotted
curve encloses the two-phase area of the pressure–volume phase diagram. The triple
line lies too close to the bottom of the diagram to be visible on this scale.
aBased on data in Ref. [ 124 ].
byFDV=n.) We repeat the steps of the derivation above, labeling the two phases by
superscriptsíandìinstead oflandg. The relation corresponding to Eq.8.2.4is
bí.FíF /Dbì.FFì/ (8.2.7)
IfLíandLìare lengths measured along the tie line from the system point to the
ends of the tie line at single phasesíandì, respectively, Eq.8.2.7is equivalent to the
general lever rule
bíLíDbìLì or
bì
bí
D
Lí
Lì
(8.2.8)

8.2.5 Volume properties

Figure8.10is a pressure–volume phase diagram for H 2 O. On the diagram are drawn
isotherms(curves of constantT). These isotherms define the shape of the three-dimensional
p–.V=n/–Tsurface. The area containing the horizontal isotherm segments is the two-phase
area for coexisting liquid and gas phases. The boundary of this area is defined by the dotted
curve drawn through the ends of the horizontal segments. The one-phase liquid area lies to