WORK OF J. N. PEARCE. 87 



If there were no hydration these values should be equal. Since the freezing- 

 point measurements give us the most accurate relation between the amount of the 

 actual solvent and the number of dissolved particles, it, therefore, must give us the 

 most accurate measure of the dissociation, when hydration is taken into account. 



The conductivity of a solution is, as we have seen, dependent upon the number of 

 ions present, their velocity, their mass and volume, and the viscosity of the solution. 

 Since the temperatures at which the dilute solutions freeze are approximately only 

 one-fourth of a degree or less below the freezing-point of pure water the tempera- 

 ture at which the conductivity measurements were made we must conclude that the 

 number of ions present and their velocities are in the two cases the same. Similarly, 

 the viscosity of the solutions is the same in both measurements, and, therefore, 

 the friction between solvent and ion will vary directly as the surface of the latter. 



We have shown that most metallic ions in solution have great hydrating power, 

 and that the degree of hydration varies inversely with the atomic volume of the 

 ion. The ions which have the greatest hydrating power are those which have the 

 smallest atomic volumes, and should, therefore, if there were no hydration, meet 

 with less friction in their movements through the solution. They should have 

 greater migration velocities, while exactly the opposite results are found. 



A comparison of the values of a for a dilute solution of the strong acids shows that 

 the dissociation as measured by conductivity is greater in every case than the dis- 

 sociation measured by the freezing-point method. In the more concentrated solu- 

 tions the observed freezing-point lowerings are higher than the molecular lowcrings 

 calculated from conductivity. This is due to the fact that the molecules or ions of 

 those acids have considerable hydrating power, and we obtain in concentrated 

 solutions results of the same character as with the salts. 



SUMMARY. 



1. The freezing-point lowerings and the conductivities of solutions of a number 

 of electrolytes, over a wide range of concentration, have been carefully redetermined. 



2. We observe that the molecular lowerings of the freezing-point of all the electro- 

 lytes studied pass through a very pronounced minimum at concentrations ranging 

 from 0.1 to 0.25 normal. The molecular lowerings calculated from the dissociation, 

 as measured by conductivity, decrease regularly from the most dilute to the most 

 concentrated solutions. 



3. The magnitude of the molecular lowerings produced by molecular quantities 

 of different salts varies directly as the number of molecules of water with which 

 those salts crystallize from solution. The magnitude of the hydrating power of 

 salts in solution is, in turn, proportional to the amount of water of crystallization. 



4. That the ions have very great hydrating power is shown by the values of M 

 and H for the different salts. The total amount of combined water decreases with 

 increase in concentration, passes through a minimum, and then increases regularly 

 with increase in concentration. The amount of water held in combination by one 

 molecule of a salt is very large in the more dilute solutions where the ions predomi- 

 nate. It decreases rapidly with decrease in dissociation, and approaches a constant 

 value at greater concentration. 



