PROPERTIES OF MATTER 



23 



evaporation is dependent on the fraction of molecules possessing veloci- 

 ties greater than the critical value allowing their escape. 



If a liquid is placed in a closed container, molecules evaporate into 

 the gas phase and move randomly about. A portion of these escaped 

 molecules wander back, strike the liquid surface, and stick to it. When 

 the rates of escape and return are equal, the system is said to be in 

 equilibrium, and net change ceases. At equilibrium there is a constant 

 number of molecules per luiit volume of gas phase, depending upon 

 both the liquid and the temperature. This constant number of mole- 

 cules exerts a pressure determined, according to the gas law, by the 

 quantity of gas. In systems of this type the term vapor pressure is 

 used and defined as the pressure exerted by a vapor in equilibrium 

 with the corresponding liquid. 



Since average velocity increases with temperature, vapor pressure in- 

 creases also, but the relationship is not linear. See Figure 2-2. Vapor 



40 60 



Temperature, "C. 



80 



FIGURE 2-2. The vapor-prcssmc curve for uiiter. Olhcr liquids possess very similar 

 curves displaced along the al)scissa and intersecting the 760-mm. line at different 

 temperatures corresponding to their particular boiling points. 



