TABLE OF CONTENTS You can swim (uncomfortably) in water at a temperature slightly above freezing; a tiny drop in temperature or a miracle allows you to walk on water.
Bohren and Albrecht
With a few exceptions, the systems that we have considered so far are perfect gases; that is, systems in which the interactions between particles can be neglected. While nitrogen and oxygen at room temperature are good examples of such gases, they constitute only a tiny fraction of the substances which we come across in Nature; furthermore, the intermolecular interaction makes all gases deviate greatly from ideal behavior at high pressure or low temperature. The most dramatic effect of this interaction is condensation of the gas to a liquid or solid. This occurs when the intermolecular attraction is strong enough, and the temperature low enough, that the energy gained by bringing the molecules into close proximity overcomes the reduction in entropy due to condensation.
We will make that general statement more precise later, but first we need to review the significance of the free enthalpy G ? U+ pV ?TS. As pointed out in Chapter 5, at a fixed temperature and volume, equilibrium is attained when the free energy F ? U ?TS is minimized. However, most processes are carried out at constant pressure rather than at constant volume; this is particularly true of phase transitions, many of which are accompanied by large changes in volume, and would produce dangerously high pressures if they were...
