Physical Chemistry Third Edition

284 6 The Thermodynamics of Solutions

160

150

Area of vapor states

Tie line area –

one liquid phase,

one vapor phase

Area of states

of one liquid

Tie line area –

one liquid phase,

one vapor phase

Tie line area –

two liquid phases

xf 5 0.478

xf 5 0.0406

xf 5 0.0917

xf 5 Furfural mole fraction

140

130

120

110

100

90

0 0.2 0.4 0.6 0.8 1.0

tC

/^8 C

Figure 6.16 Liquid–Vapor Temperature–Composition Phase Diagram of Furfural

and Water at 1.000 atm.After G. H. Mains,Chem. Met. Eng., 26 , 779 (1922).

between the liquid and vapor phases disappears as the liquid–vapor critical point is

approached, as was shown in Figure 1.6.

There are a few mixtures, such as water and nicotine, that have both an upper and

a lower consolute point, so that the boundary of the tie-line region is a closed curve.

Below the lower consolute point at 61.5◦C water and nicotine mix in all proportions.

Above the upper consolute temperature at 233.0◦C they also mix in all proportions.^9

Between these temperatures there is a tie-line region in the diagram and the liquids

are only partially miscible. Another mixture with both a lower and an upper consolute

temperature is butoxyethanol and water, with a lower consolute temperature of 48.01◦C

and an upper consolute temperature of 130.7◦C.^10

If the positive deviation from ideality is even greater than that of Figure 6.15, the

two-phase region can extend to the liquid–vapor region and produce a phase diagram

like that of Figure 6.16, which shows the temperature–composition phase diagram

of furfural and water at a constant pressure of 1.000 atm. The horizontal tie line at

(^9) A. N. Campbell, E. M. Kartzmark, and W. E. Falconer,Can. J. Chem., 36 , 1475 (1958).
(^10) Y. Izumiet al.,J. Physique, 42 , 544 (1981); H. L. Cox and L. H. Cretcher,J. Amer. Chem. Soc., 48 , 451
(1926).