THEOR\ OF CONCENTRATED SOLUTIONS. 



133 



If mixtures of two given liquids are found to obey the 

 straight line law, then by analogy with gaseous mixtures we 

 conclude that the molecules of the two components are in the 

 same state after mixing, as before. If, however, the property- 

 composition curve is " sagged " or otherwise different from a 

 straight line, then some molecular change must have taken place on 

 mixing. To fix our ideas, let us consider the density-composition 

 curve of a mixture of two liquids. If the curve is always below 

 the straight line, i.e., the density always less than its theoretical 



Composition of mixture. 

 Fig. 1. 



value, the usual explanation is that dissociation of one of the 

 components has taken place. Similarly, if the density were 

 always greater than the theoretical value as deduced from the 

 law of mixtures, this would generally be taken to indicate the 

 association of the molecules of one of the liquids. In short, the 

 only deductions that one can make from these curves are in 

 general deductions by analogy with the better-known gaseous 

 systems. The occurrence of a maximum in the property- 

 compoHtion curve has usually been taken to represent chemical 



