Physical Chemistry Third Edition

5.3 Phase Equilibria in One-Component Systems 207

water on the earth except in very hot regions freezes to this form of ice, destroying

life as we know it. In the late 1960s, it was thought for a time that liquid water might

exhibit polymorphism. It was claimed there was a second liquid phase of water, which

was namedpolywatersince it seemed to consist of polymers of water molecules.^4

This phase appeared to have a lower chemical potential than ordinary liquid water at

the same temperature and pressure. Numerous experimental and theoretical studies of

polywater were published before it was discovered that the small capillaries in which the

polywater was supposedly prepared were leaching substances into the water, forming

solutions. If it had been a real phase, polywater would have threatened life just as did

the fictional form of ice in Vonnegut’s novel.

Helium has different phase diagrams for the two principal isotopes, and both isotopes

exhibit liquid-phase allotropy. Figure 5.4 shows the low-temperature phase diagrams

of^4 He and^3 He. The diagram for^4 He (the more abundant isotope) shows two different

liquid forms, called helium I and helium II. There are two triple points, one for the two

liquid forms and the vapor phase, and one for the two liquid forms and the solid phase.

The diagram for^3 He shows three different liquid phases and three triple points. Neither

isotope exhibits coexistence between the solid and the vapor, and the solid phases of

both isotopes can exist only at pressures larger than 1 atm. Helium is apparently the

only substance that cannot be frozen at 1 atm pressure.

40 (a)

30

20

10

Solid^4 He

He ll

Lambda

line He l

He vapor Criticalpoint

0

P

/ atm

T/K

40 (b)

30

20

10

Liquid

(^3) He-A
Liquid
(^3) He-B
(^3) He vapor
Normal liquid^3 He
Solid^3 He
Critical
point
0
1023 1022 10 1
Figure 5.4 The Low-Temperature Phase Diagrams of^3 He and^4 He.From P. V. E.
McClintock, D. J. Meridith, and J. K. Wigmore,Matter at Low Temperatures, Wiley, New York,
1984, p. 18.
(^4) E. R. Lippincottet al.,Science, 164 , 1482 (1969); A. Cherkin,Nature, 224 , 1293 (1969).