Of state. Contents and Concepts 1. Comparison of Gases, Liquids, and Solids Changes of State

Clausius–Clapeyron Equation: Relating Vapor Pressure

and Liquid Temperature

We noted earlier that the vapor pressure of a substance depends on temperature. The variation of vapor pressure with temperature of some liquids was given in Figure 11.7.

11 States of Matter; Liquids and Solids

It can be shown that the logarithm of the vapor pressure of a liquid

or solid varies with the absolute temperature according to the following

a pproximate relation:

H ere ln P is the natural logarithm of the vapor pressure, and A and B are positive constants. You can confirm this relation for the liquids shown in Figure 11.7 by replotting the data. If you put y = ln P and x = 1/T, the previous relation becomes

This means that if you plot ln P versus 1/T, you should get a straightline with slope -A. The data of Figure 11.7 are replotted this way in Figure 11.10. <

The previous equation has been derived from thermodynamics, by assuming the vapor behaves like an ideal gas. The result, known as the Clausius–Clapeyron equation, shows that the constant A is proportional to the heat of vaporization of the liquid, _H_vap.


FIGURE 11.10