Physical Chemistry Third Edition

1.4 The Coexistence of Phases and the Critical Point 27

1.4 The Coexistence of Phases and the

Critical Point

Transitions from a gaseous state to a liquid state or from a liquid state to a solid state,

and so forth, are calledphase transitionsand the samples of matter in the different

states are calledphases. Such transitions can take place abruptly. If a gas is initially

at a temperature slightly above its condensation temperature, a small decrease in the

temperature can produce a liquid phase that coexists with the gas phases, and a further

small decrease in the temperature can cause the system to become a single liquid phase.

This remarkable behavior is an exception to the general rule that in nature small changes

produce small effects and large changes produce large effects.

It is an experimental fact that for any pure substance the pressure at which two phases

can coexist at equilibrium is a smooth function of the temperature. Equivalently, the

temperature is a smooth function of the pressure. Figure 1.4 shows schematic curves

representing these functions for a typical substance. The curves are calledcoexistence

curvesand the figure is called aphase diagram. The three curves shown are the solid–

gas (sublimation) curve at the bottom of the figure, the liquid–gas (vaporization) curve

at the upper right, and the solid–liquid (fusion, melting, or freezing) curve at the upper

left. The three curves meet at a point called thetriple point. This point corresponds to

Liq

uid

va

po

rc

oe

xis

te

nc

e

cu

rv

e

Triple point

Solid–vapor

coexistence curve

T

Normal freezing temperature

Normal boiling temperature

Solid

(3-Step process to liquefy gas

without phase transition)

Cooling at

constant pressure

Critical

point

Expansion

at constant

temperature

Compression

at constant

temperature

Liquid Vapor (gas)

1 atm

P

Solid–liquid coexistence curv

e

Figure 1.4 The Coexistence Curves for a Typical Pure Substance (Schematic).