THEORY OF CONCENTRATED SOLUTIONS. 



141 



The effect of temperature change on property-composition and on 

 deviation-composition curves. 



It has been frequently observed that when the property 

 composition curve for a binary mixture exhibits a maximum, the 

 composition at which this maximum occurs is not fixed, but 

 changes with temperature alteration. It has been pointed out 

 that it was this fact which has led many chemists to reject the 

 idea of determining the presence of compounds in solution from 

 a study of such property-composition curves. Such an attitude 

 is, of course, correct, since one cannot imagine a compound which 

 changes its actual composition gradually with change of 

 temperature. 



Xow from what has been already said, it is evident that the 

 actual form of the property-composition curve for a given binary 



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Fig 5. 



mixture wherein chemical combination has taken place depends 

 on the relative amounts of the three molecular species present, 

 and also on the separate properties of these. Change of tem- 

 perature will almost always alter the equilibrium constant K. 

 that is, K will alter the relative amounts of A, B, and ABn in the 

 system. Moreover each molecular species has its own tem- 

 perature co-efficient for any physical property, and therefore it 

 is only to be expected that change of temperature will alter the 

 position of. and may even cause the disappearance of a maxi 

 mum in the actual property-composition curve. It is, however, 

 quite different with the deviation curve, i.e., the curve obtained 

 by plotting the deviations from the mixture law against the com- 

 position of the mixture. The position of maximum deviation 

 depends only on the formula of the compound formed, and is 



