Physical Chemistry Third Edition

206 5 Phase Equilibrium

This function is represented by acoexistence curvein the phase diagram. Figure 1.4

shows a solid–vapor curve, a solid–liquid curve, and a liquid–vapor curve. The equi-

librium pressure when a liquid phase or a solid phase is equilibrated with a vapor (gas)

phase is called thevapor pressureof that phase. Figure 5.2 shows the equilibrium vapor

pressure of liquid water as a function of temperature.

0

0

100

200

300

400

P (torr)

500

600

700

20 40 60

T(°C)

80 100

Figure 5.2 The Equilibrium Vapor
Pressure of Water as a Function of
Temperature.Data from R. C. Weast,
Handbook of Chemistry and Physics,
64th ed., CRC Press, Boca Raton, FL,
p. D192.

If one component and three phases are present,f0 and there is no choice about

the temperature, the pressure, or any other intensive variable. This three-phase state is

represented by atriple pointat which three coexistence curves intersect. The solid–

liquid–vapor triple point of water occurs at a temperature of 273.16 K (this value defines

the size of the kelvin) and a pressure of 4.562 torr.

Figure 5.3 shows the phase diagram of water. Water exhibits polymorphism, so there

are several coexistence curves. The pressure scale in this diagram is so compressed that

the liquid–vapor curve of Figure 5.2 is too close to the horizontal axis to be visible. Eight

different equilibrium crystal forms of water are shown, denoted by Roman numerals.

Ice IV is a metastable phase that was mistaken for an equilibrium phase when it was

given this number.^2 We do not count it as one of the equilibrium crystal forms of water.

In the novelCat’s Cradle^3 a fictional form of ice is discovered that melts at 114◦F.

Since it is more stable than liquid water at room temperature, ultimately all of the

0

0

4 IX

VI 9

VI

VlI

V

III

ll

I

VIII

8

P/ kbar^12

16

20

24

2160 2120 280 240 0 40

Liquid

tC/(°C)

80 120

Figure 5.3 The Phase Diagram of Water.From B. Kamb, in E. Whalley, S. Jones, and

L. Gold, eds.,Physics andChemistry of Ice, University of Toronto Press, Toronto, 1973.

(^2) Professor Martin Chaplin of London South Beach University maintains a website on the properties of
water, incuding metastable ice IV. Since the web address of the site can change, the site is best accessed by
searching for Professor Chaplin’s name.
(^3) Kurt Vonnegut,Cat’s Cradle, Delacorte Press, New York, 1963.