GENERAL DISCUSSION OF RESULTS. 143 



This failure of the gas laws to apply to even fairly concentrated solu- 

 tions was held up by the early opponents of the theory of electrolytic 

 dissociation as a weak point in this generalization as a general theory of 

 solutions, and it must be confessed with some justice. We had here a 

 theory of solutions which applied quantitatively only to ideal solutions, 

 and did not accord with the facts for a single solution of even moderate 

 concentration. Further, there was no reasonable explanation offered to 

 account for this failure. In the case of gases, Van der Waal's equation 

 adapted the simpler gas laws even to fairly concentrated gases, but in the 

 case of solutions there was apparently no way to account for these dis- 

 crepancies between the facts and theory; and thus the matter stood for 

 quite a time. 



The theory of electrolytic dissociation said simply this, that when 

 molecules of acids, bases, and salts are brought into the presence of water, 

 they are broken down into ions, to a greater or less extent depending upon 

 the concentration of the solution. It did not recognize any combination 

 of the solvent with the dissolved molecules or the ions. 



The dilution of the solution was determined by the amount of dis- 

 solved substance in a given volume of the solution, assuming that all of 

 the liquid present ivas acting as solvent. 



We now know that this is not the case. A part of the solvent is, in 

 most cases, combined with the dissolved substance and is not playing the 

 role of solvent. There is, therefore, less solvent present than was supposed, 

 and this is the same thing as to say that the solution is more concentrated 

 than it was thought to be from the way in which it was prepared. 



The amount of the combined solvent may be small, as in the cases where 

 the dissolved substances do not crystallize with any of the solvent of 

 crystallization. It may, however, be very large, as in the case of aluminium 

 chloride in water at what is supposed to be twice normal concentration. 

 Here about four-fifths of the water present is in combination with the 

 dissolved substance, and the solution is really about five times as concen- 

 trated as would be supposed from the amount of the salt present in a given 

 volume. 



We thus see a reason for the failure of the gas laws to hold for con- 

 centrated solutions. In dilute solutions the amount of water combined 

 with the dissolved substance, as compared with the total amount of water 

 present, is practically negligible. In more concentrated solutions, however, 

 the amount of the combined water may be a very appreciable part of the 

 total water present, or in extreme cases, as that of aluminium chloride cited 

 above, it may be several times the water that is present acting as solvent. 

 Concentrated solutions are thus more concentrated than we woidd suppose 

 without the theory of solvation, and this accounts for the failure of the gas 

 laws to apply to such solutions. 



A theory to be of greatest scientific value must, of course, be quanti- 

 tative. While we have not been able, up to the present, to determine 

 accurately the magnitude of the hydration in aqueous solutions, yet the 

 approximate composition of the hydrates formed by a large number of 

 substances at various concentrations has been worked out; so that the 



