THE PRESENT POSITION OF THE 

 HYDRATE THEORY OF SOLUTIONS 



By T. MARTIN LOWRY, D.Sc. 

 Lecturer in Physical Chemistry and Crystallography at the Central Technical College 



PART I 



It is characteristic of the wide difference of opinion that has 

 existed in reference to the nature of solutions in general, and 

 of aqueous solutions in particular, that the editor of Watt's 

 Dictionary of Chemistry found it advisable in 1894 to include 

 articles on " Solutions " by two different authors. The theory 

 described by Arrhenius in the former of the two articles was 

 essentially one of dissociation, it being supposed that the main 

 effect of adding salt to water was the resolution of the salt 

 into the ions or charged radicles which separate at the electrodes 

 when the salt is decomposed by electrolysis. Pickering, develop- 

 ing in the second article a view of the nature of solutions pre- 

 viously elaborated by Mendeleef, urged that the main effect of 

 bringing together the solvent and solute was to initiate a 

 process of association whereby the salt and water combined 

 together to form a series of hydrates, the complexity of which 

 increased with the proportion of water added to the salt. 



That the action of the solvent was incomplete and reversible 

 appears to have been recognised, at least to some extent, in 

 each case. Arrhenius' theory, which may be represented 

 symbolically by the equation, 



NaCl~^Na+ CI 



included as one of its essential features the idea of a " co- 

 efficient of ionisation," which showed the extent to which the 

 salt had become ionised or dissociated in solution. As the 

 conductivity of the electrolyte was attributed wholly to the 

 ionised portion of the salt, and ionisation was thought to be 

 complete at infinite dilution, it was easy to deduce a value for 

 this co-efficient by comparing the molecular conductivity of the 



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