THEORY OF CONCENTRATED SOLUTIONS. 1,^5 



denote by AB. As the proportions of A and B are altered, the 

 amounts of AB formed must vary according to the law govern- 

 ing homogeneous equilibria, namely, the law of mass action. 



It may be objected that A and B may form a number of 

 compounds in solution, such as AB, AB.,, AB,, .... Such 

 a supposition gives us. of course ( with regard to aqueous solu- 

 tions), the older hydrate theory of Mendeleef, Pickering, and 

 others. 



It seems to be that the theory of an indefinite number of 

 hydrates or similar compounds in equilibrium with the two com- 

 ponents is very improbable. Each such hydrate will have a 

 definite dissociation tension which varies with the temperature, 

 pressure, and its concentration. The aqueous vapour tension 

 of the hydrate must be in equilibrium with the total aqueous 

 vapour pressure of the system, and if the dissociation pressures 

 of the different hydrates differ widely, as one would expect, 

 then their amounts (in the presence of the two components) 

 must also differ very widely. Thus, in the presence of the two 

 components, A and B, one compound must at any rate very 

 largely predominate. The matter becomes clearer if we consider 

 an analogous gaseous equilibrium. Imagine a mixture of 

 nitrogen and oxygen at such a temperature that true equilibrium 

 is possible. Such a condition is easily attainable in practice, and 

 the system has been the subject of considerable experimental 

 study. It is not found to be possible for all the oxides of nitro- 

 gen to exist simultaneously in equilibrium with the two com- 

 ponents N, and O a , nor woidd any chemist expect this . from 

 a priori considerations. We can, of course, obtain systems con- 

 sisting of X„ 0. 2 and NO or NO, 2 , and N0 2 , or N0 2 , O, and 

 N 2 O in equilibrium according to the conditions of temperature 

 and pressure. The reasons why the various oxides cannot simul- 

 taneously co-exist in equilibrium with the two components is that 

 the different compounds have widely varying energy contents. 

 In other words, the dissociation tendencies of the various oxides 

 are so very different that their co-existence in the presence of 

 N„ and O., is impossible except in infinitely small amounts. ( )ne 

 compound largely predominates, but which one it is depends upon 

 the conditions. Should one of the components disappear, then 

 another compound may be formed, and we may thus have NO, 

 NO, and O, in equilibrium. .Again, the amounts of each will so 

 adjust themselves that their oxygen dissociation pressures are in 

 equilibrium with the total oxygen pressure of the system. 



Whether any flaw be found in this reasoning or not, the fact 

 remains that if we allow the co-existence of any number of 

 hydrates or similar compounds in a homogeneous liquid system 

 in the presence of the two components, then the system becomes 

 incapable of mathematical treatment on the lines of the law of 

 mass action, and no quantitative theory of solution is likely on 

 such a basis. To my mind this is precisely the weakness of the 



