SOLUTION 117 



ceedingly good conductor. The significance of these differences 

 in behavior will be taken up later (p. 118). 



In the case of many properties of solutions, however, it has 

 been found that equal numbers of dissolved molecules of different 

 substances produce the same amount of change. The effect appears 

 here to be due essentially to physical causes, and is discussed in 

 the following sections in the light of the molecular hypothesis. 

 Before attacking these sections, the student is recommended to 

 refer back to p. 62-4 and read these pages through again care- 

 fully, noting that the equilibrium relationships between liquid 

 water and water vapor, therein discussed, can obviously be ex- 

 tended to any volatile substance in contact with its own vapor. 



Vapor Pressure of Solutions. When we take equal quan- 

 tities of a volatile liquid (e.g., benzene, CeHe) and add to each 

 equal weights of different non-volatile solutes (e.g., naphthalene, 

 anthracene, camphor; three organic solids which are practically 

 non-volatile at ordinary temperatures) we find that the vapor 

 pressures of all the resulting solutions are less than that of the 

 pure solvent, but the depression is different in each case. But if, 

 instead of adding equal weights of the different solutes, we add 

 equal numbers of molecules (as we can do by dissolving, for example, 

 1 g. molecular weight of each substance in 1000 g. of benzene), 

 we find that the depression is the same in every case. The depres- 

 sion is proportional, moreover, to the fraction of solute molecules 

 in the solution. This very striking fact is explained by the mo- 

 lecular hypothesis as follows. 



Every molecule at the surface of a pure volatile liquid has an 

 equal chance to escape into the vapor above the liquid. But 

 as soon as we add to such a liquid a solute which is practically 

 non-volatile, we have a liquid in which some of the molecules 

 have no tendency to pass into the state of vapor, but are fixed 

 in the liquid state. Suppose, for instance, we consider a solution 

 in which one molecule in every ten is non-volatile; the intensity 



