42 LIBERATION OF ENERGY 



According to the kinetic theory the second and third classes have 

 a larger or smaller number of particles in solution than theory 

 warrants. 



Where have the extra particles in Class 2 come from ? Has 

 the molecule divided ? If one compares the osmotic pressure of 

 cane sugar and sodiimi chloride in gram-molecular solution, 

 one finds them (roughly) as 1 : 2. How can this be explained ? 

 In 1887 Arrhennius propounded his dissociation theory, which has 

 since been proved, amplified and universally accepted. According 

 to this theory, some of the molecules of certain salts when dissolved 

 in water split up or dissociate into their constituent ions. An ion 

 is an atom or a sub-molecular group charged with electricity and 

 attached to certain water molecules. 



For example, 



NaCl (solid) + aq = cat-ion + an-ion 



= Na + CI 



= (Na(H20)ci') + {C\{B.^O)ij). 



It is the presence of these ions which gives a solution the power 

 of conducting electricity, and so any substance which dissociates, 

 i.e. becomes ionised on going into solution, is said to be an electro- 

 lyte (Chap. VII.). 



It is worthy of note that electrical conductivity is not a property 

 either of the solvent or of the solute, but of the solution. (Part II.) 



All electrolytes are not dissociated to the same extent. A salt 

 of either a strong acid or a strong base requires the addition of 

 comparatively little water completely to convert all its molecules 

 into ions. On the other hand, a weak acid or base is difficult to 

 dissociate. If the gram-molecular weight of an electrolyte be 

 dissolved in a litre of water a certain fraction of the molecules will 

 split into ions. This fraction is the degree of ionisation of the 

 electrolyte at this concentration. The degree of ionisation may be 

 determined by estimating the amount of resistance of the solution 

 to a small electric current (conductivity method), or it may be 

 approximately calculated from the lowering of the freeezing point. 

 (For univalent strong electrolytes at concentrations not exceeding 

 0-1 molar, the error of this determination is in most cases between 

 1 and 4 per cent.) 



One may note in passing that the ions of many electrolytes 

 possess the property of uniting with other ions, or even with 

 non-electrolytes in solution, to form complex ions. For ex- 

 ample, ions cannot normally remain free in aqueous solution, 

 but become hydrated. A hydrated ion is sometimes termed an 

 ionic micelle. 



