THE ELECTROLYTIC DISSOCIATION THEORY. 83 



other. Thus, if to a dilute solution of NaCl a solution of 

 silver nitrate be added we get a precipitate of silver chloride, 

 because this substance, being insoluble in water, cannot under- 

 go ionisation. The action which occurs is to oe regarded 



+ 

 thus : Both solutions contain chiefly ions, the one Na and Cl, 



+ 

 the other Ag and N0 ;! . On mixing the solution a precipitate of 



insoluble AgCl separates owing to the union of the Ag and Cl 



ions, and the solution now contains not NaNO 3 , as is usually 



+ 

 taught, but really ions Na and N0 3 . On evaporation these 



ions unite and form NaN0 3 , which may be obtained in crystals. 

 All the changes which take place when aqueous solutions of 

 salts are mixed are to be thus represented. The most difficult 

 conception in connection with the theory is that the ions have 

 not the usual properties of the free substances themselves, 

 which in many cases could not exist in contact with water, 

 but, because of their electrical charges, are utterly different. 

 When the electrical charge is removed, as by electrolysis, the 

 ions disappear and the material of which they are composed 

 takes its usual properties. 



Thus, to consider common salt solution, the Na ions possess 

 none of the properties of sodium nor the Cl ions any of those 

 of chlorine ; but when a current of electricity is passed the Na 

 ions are freed from their charge and metallic sodium is set 

 free at one pole (cathode), and, being in contact with water, at 

 once reacts, giving free hydrogen and caustic soda, while at 

 the other pole (anode) the Cl ions lose their charge of negative 

 electricity and are evolved as chlorine gas. 



When a salt is dissolved in water, it may partially dissolve 

 as such, that is, some molecules of the original salt dissolve ; 

 but decomposition into ions at once commences and goes on 

 until there is a certain fixed ratio (for the particular salt and 

 dilution) between the undecomposed salt and the products of 

 the ions present. Take common salt, for example. The 

 equilibrium will be reached when 



k (number of mols. of undecomposed NaCl) 

 = (number of Na ions x number of Cl ions) 



(k being a constant which varies with the dilution and 



G2 



