Concise Physical Chemistry

c02 JWBS043-Rogers September 13, 2010 11:23 Printer Name: Yet to Come

26 REAL GASES: EMPIRICAL EQUATIONS

root of the subcritical isotherm is not observed experimentally but has theoretical

significance.

The constant pressure horizontal that intersects the subcritical isotherm represents

vaporization going from left to right or condensation going from right to left. It is

called anisobar. In vaporization, the volume of the system gets much larger but the

pressure stays the same. Along an isobar, liquid and vapor are in equilibrium. If the

pressure indicated by the isobar is atmospheric pressure, the subcritical isotherm is

at the normal boiling pointTb.

2.4.2 The Critical Density

Below the critical temperature, one can measure the densities of a gas and a liquid at

equilibrium in a closed container. Upon raising the temperature slightly at constant

pressure, the density of the gas increases because liquid vaporizes. The density of

the liquid decreases due to expansion. These densities, one increasing and the other

decreasing, must approach each other as in Fig. 2.6. At the point where they meet,

the densities of the gas and liquidand all other propertiesbecome identical. This is

the critical temperature. Determination of the critical temperature of O 2 by density

measurements is shown in Fig. 2.6.

T (K)

115 120 125 130 135 140 145 150 155

Density

0

1

.

153.5

FIGURE 2.6 Densityρcurves for Liquid and Gaseous Oxygen. The straight line represents

the arithmetic mean density of the liquid and gas (ρl+ρg)/2. The three curves meet atT=

153.5 K. The tabulated value forTcis 154.6 K (CRC Handbook of Chemistry and Physics

2008–2009, 89th ed.).