The Foundations of Chemistry

PHASE DIAGRAMS (PVERSUS T)

We have discussed the general properties of the three phases of matter. Now we can

describe phase diagramsthat show the equilibrium pressure–temperature relationships

among the different phases of a given pure substance in a closed system. Our discussion

of phase diagrams applies only to closed systems(e.g., a sample in a sealed container), in

which matter does not escape into the surroundings. This limitation is especially impor-

tant when the vapor phase is involved. Figure 13-17 shows a portion of the phase diagrams

for water and carbon dioxide. The curves are not drawn to scale. The distortion allows

us to describe the changes of state over wide ranges of pressure or temperature using one

diagram.

The curved line from Ato Cin Figure 13-17a is a vapor pressure curve obtained exper-

imentally by measuring the vapor pressures of water at various temperatures (Table 13-8).

Points along this curve represent the temperature–pressure combinations for which liquid

and gas (vapor) coexist in equilibrium. At points above AC,the stable form of water is

liquid; below the curve, it is vapor.

13-13

13-13 Phase Diagrams (Pversus T)507

TABLE 13-8 Points on the Vapor Pressure Curve for Water

temperature (°C) 10 0 20 30 50 70 90 95 100 101

vapor pressure (torr) 2.1 4.6 17.5 31.8 92.5 234 526 634 760 788

0.01 374

2.1

4.6

355

218 atm B C

A

D

Gas

Liquid

water, H 2 O

Temperature (°C)

Pressure (torr)

(a)

B

1

5

10

73 atm

–78 –57 0 31

C

Gas

Liquid

carbon dioxide, CO 2

Solid

5.2 atm

Temperature (°C)

Pressure (atm)

(b)

Figure 13-17 Phase diagrams (not to scale). (a) Diagram for water. For water and a

few other substances for which the solid is less dense than the liquid, the solid–liquid

equilibrium line (AB) has negative slope, that is, up and to the left. (b) Diagram for carbon

dioxide, a substance for which the solid is denser than the liquid. Note that the solid–liquid

equilibrium line has positive slope, that is, up and to the right. This is true for most

substances.

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 13.17, Phase Diagrams.