THE FORMATION OF CHEMICAL COMPOUNDS IN 

 HOMOGENEOUS LIQUID SYSTEMS: A CONTRIBU- 

 TION TO THE THEORY OF CONCENTRATED 

 SOLUTIONS. 



By Prof. Robert Beckett Denison, M.A., D.Sc, Ph.D. 



The question whether chemical combination between two 

 liquids can be detected, and, if so, whether the formula of the 

 compound formed can be obtained from a consideration of the 

 physical properties of mixtures of the two liquids, is probably 

 one of the most controversial points in the whole of theoretical 

 chemistry. At the same time, it is an exceedingly important 

 question, since, if it can be answered in the affirmative, we are 

 at once in the possession of a simple method of determining the 

 formulae of compounds in solution without their isolation being 

 necessary. 



With regard to the physical properties of gaseous mixtures, 

 it is well known that the simple mixture law is generally obeyed. 

 That is, for all mixtures of two gases from o to ioo per cent, 

 of either constituent, the value of any physical property is inter- 

 mediate between, and lies on a straight line joining the values 

 of the physical property for the two pure gases. 



Taking as example the density of mixtures of oxygen and 

 nitrogen, we have roughly = i6 and N = i4 (H=i). Plotting 

 the fractional composition by volume as abscissa, and the density 

 as ordinate, then the densities o fall mixtures of nitrogen and 

 oxygen lie on the straight line NO. It is almost unnecessary to 

 point out that in order to obtain this straight line relationship 

 in general, the composition must be expressed as fractional com- 

 position by volume when the physical property has volume in 

 the denominator, e.g., 



mass 



— , ^density. 



volume J 



Conversely, composition must be expressed by weight if 

 the property has weight in the denominator, e.g., 



Specific volume=the volume of unit weight. 



Instead of plotting the fractional composition by volume, we 

 can replace it by the fractional number of molecules, since in the 

 case of gases Avogadro's hypothesis holds good. In short, the 

 specific gravity-composition curve of a gaseous mixture is a 

 straight line whether the composition be plotted as molar- 

 fraction or as fractional composition by volume. 



This simple form of what we will term property-composition 

 curves is not shown by binary liquid mixtures, except in rare 

 instances. The usual forms are depicted in Fig. i, curves 

 2. 3, and 4. 



