44 ENERGY OF SUBSTANCE IN SOLUTION 



1. O.P. Approximately 22-4 atmos. 



2. O.P. Decidedly greater than 22-4 ,, 



3. O.P. less 22-4 



The first class have been termed simple (undissociated) solu- 

 tions. According to the kinetic theory the second and third 

 classes have a larger or smaller number of particles in solution 

 than theory warrants. 



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

 the molecule divided ? If one compares the osmotic pressure 

 of cane sugar and sodium chloride in gram-molecular solution, 

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

 In 1887 Arhennius propounded his dissociation theory, which has 

 since been proved, amplified and universally accepted. Accord- 

 ing 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 -fan-ion 



=Na+Cl 

 = (Na(H 2 0)*)+(Cl(H 2 0)?/). 



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 elec- 

 trolyte. 



All salts may therefore be classed as 

 Electrolytes, Class 2. 

 Non-Electrolytes, Class 1 and Class 3. 



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

 either of the solvent or of the solute, but of the solution. 



Pure distilled water is a non-electrolyte. 



Pure dry hydrochloric acid is a non-electrolyte. 



But an aqueous solution of hydrochloric acid is a very good 

 electrolyte. 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 



