The Equilibrium Nature of Phase Changes

In everyday experience, phase changes take place in open containers—the outdoors, a pot on a stove, the freezer compartment of a refrigerator—so such a change is not reversible. In a closed container under controlled conditions, however, phase changes of many substances are reversible. 

B Molecules enter and leave liquid at same rate. 

In a closed flask at constant temperature with the air removed, the initial pressure is zero. As molecules leave the surface and enter the space above the liquid, the pressure of the vapor rises. 

At equilibrium, the same number of molecules leave as enter the liquid within a given time, so the pressure of the vapor reaches a constant value. C, A plot of pressure vs. time shows that the pressure of the vapor increases as long as the rate of vaporization is greater than the rate of condensation. At equilibrium, the rates are equal, so the pressure is constant. The pressure at this point is the vapor pressure of the liquid at that temperature. 

CHAPTER 12 Intermolecular Forces Liquids, Solids, and Phase Changes 

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