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

FIGURE 11.4

Measurement of the vapor pressureof water

(A) The mercury column drops a distance h because of the vapor pressure of water. (B) A detail of the upper portion of the

tube, showing water molecules evaporating from and condensing on the water surface.

11.2 Phase Transitions

FIGURE 11.5

Distribution of kinetic energies ofmolecules in a liquid

Distributions of kinetic energies are shown for two different temperatures. The fraction of molecules having kinetic energies greater than the minimum necessary for escape is given by the colored areas (orange for the lower temperature, red area plus orange area at the higher temperature). Note that the fraction of molecules having kinetic energies greater than the minimum value for vaporization increases with temperature As molecules at the surface gain sufficient kinetic energy (through collisions with neighboring molecules), they escape the liquid and go into the space at the top of the barometer column (see Figure 11.4B). More and more water molecules begin to fill this space. As the number of molecules in the vapor state increases, more and more gaseous molecules collide with the liquid water surface, exerting pressure on it. So, as vaporization of water proceeds, the mercury column moves downward. Some of the molecules in the vapor collide with the liquid surface and stick; that is, the vapor condenses to liquid. The rate of condensation steadily increases as the number of molecules in the volume of vapor increases, until the rate at which molecules are condensing on the liquid equals the rate at which molecules are vaporizing (Figure 11.6).

FIGURE 11.6