Physical Chemistry , 1st ed.

exist as a definite liquid or gas. (It can exist as a solid if the temperature is low

enough.) The state of the H 2 O is called supercritical.Supercritical phases are

important in some industrial and scientific processes. In particular, there is a

technique called supercritical fluid chromatography in which compounds are

separated using supercritical CO 2 or other compounds as a “solvent.”(TCand

pCfor CO 2 are about 304 K and 73 bar.)*

One other point in the phase diagram is worthy of mention. Figure 6.5 in-

dicates a set of conditions where solid, liquid, and gas are in equilibrium with

each other. This is called the triple point.For H 2 O, the triple point is 0.01°C,

or 273.16 K, and 6.11 mbar, or about 4.6 torr. Because H 2 O is so common, the

triple point for H 2 O is recognized internationally as a verifiable temperature

standard. All materials have triple points, a unique set of pressure and tem-

perature conditions in which all three phases can exist in equilibrium with

each other. Table 6.3 lists conditions of critical points for some substances.

The phase diagram for H 2 O is commonly used as an example for several

reasons: it is a common material, and the phase diagram shows some unusual

characteristics. Figure 6.6 shows a more expansive phase diagram for the

156 CHAPTER 6 Equilibria in Single-Component Systems

*One method of decaffeinating coffee beans is by using supercritical CO 2.

Ice VI

Ice V

Ice I

Ice III

Ice I

6000

0

200

Temperature (K)

700

Ice

II

Pressure (bar)

4000

2000

218

1

2

0.006

600

647.30

400 500

273.15 273.16 373.15

300

Critical point

Triple

point

Liquid (water)

Gas (steam)

Liquid (water)

Figure 6.6 This phase diagram of water extends to higher temperatures and pressures than

Figure 6.3. Notice that there are several possible crystal structures of solid H 2 O, most of which

exist only at high pressures. Two forms of solid H 2 O have only recently been discovered.

Table 6.3 Critical temperatures and

pressures for various substances

Substance TC(K) pC(bar)

Ammonia 405.7 111

Hydrogen 32.98 12.93

Methane 191.1 45.2

Nitrogen 126 33.1

Oxygen 154.6 50.43

Sulfur 1314 207

Water 647.3 215.15