284 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



number and the distribution of the charges within these complexes. If 

 these complexes are relatively stable, as we know the ion complex to be, 

 then the properties of the complexes will be relatively independent of the 

 manner in which two or more of them are grouped together. We should 

 not, therefore, expect any considerable change in those properties of elec- 

 trolytes which depend primarily upon the distribution of the charges on 

 the ions; for the ionic complexes exist practically unchanged in the 

 un-ionized molecules whatever their state; that is, whether in solution or 

 as liquid, solid, or, perhaps, even vapor. Only such properties as depend 

 on the field due to the ions may be expected to exhibit a marked differ- 

 ence for the ions and the un-ionized molecules. In the un-ionized state 

 the two ions form an electrical doublet with a closed field, while in the 

 ionized state the field is open. Those properties, therefore, which depend 

 upon the field in the immediate neighborhood of the ions should give evi- 

 dence of the existence of the ions and of the un-ionized molecules, should 

 these molecules be present in solution. 



Foremost among the properties of this class we should expect the den- 

 sity of solutions to be included. It is well known that the solution of 

 salts in water is accompanied by a marked volume contraction, which is 

 the greater the lower the concentration of the solution. According to 

 Drude and Nernst, 5 a volume change is to be expected as a result of 

 the action of the ionic charge on the molecules of the surrounding 

 medium. Obviously, other effects may come into play, such as the hydra- 

 tion of the ions, etc. 



The density of aqueous solutions has been studied from this point of 

 view by Heydweiller. 6 He found that, with a few exceptions, the density 

 change of electrolytic solutions may be represented as a linear function 

 of the ionization corresponding to Equation 82. It is true that the pre- 

 cision of the density measurements is not always great and often the 

 concentration range over which the equation has been tested is not large. 

 Then, again, the lowest concentrations up to which the relation has been 

 tested is not much below 0.1 N. It is a remarkable fact, however, that 

 for a number of electrolytes the density may be expressed as a linear 

 function of the ionization over large concentration ranges, as, for example, 

 in the case of zinc chloride, calcium chloride and potassium hydroxide. 



The constant B is the equivalent percentage density change due to the 

 un-ionized salt. If it be assumed that the un-ionized molecules in the 

 solution occupy the same volume as they do in the pure condition as salts, 

 then the value of the constant B may be calculated from the known den- 



Drude and Nernst, Ztschr. f. pJiys. Chem. 15, 79 (1894). 



Heydweiller, loc. cit. 



