78 IONIC THEOEY 



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 decom- 

 position into ions at once commences and goes on until there is a 

 certain fixed ratio (for the particular salt and dilution) between the un- 

 decomposed salt and the product of the number of 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 temperature, 

 the numbers being those present in unit volume of the solution). 



If another salt, having ions of a similar kind, be dissolved in the 

 same solution, the equilibrium previously existent is disturbed. Suppose, 

 for example, sodium sulphate were added to the common salt solution, 



+ + 



The ions of the new salt are Na, Na, and S0 4 , and the conditions of 

 equilibrium are determined by the equation 



k (Na 2 S0 4 ) = Na x Na x S0 4 



as before, k, of course, having another value ; but in the mixture the 

 Na ions are partly furnished by the common salt, partly by the 

 sodium sulphate. The consequence is that inasmuch as the degree of 

 dissociatioQ of each salt depends partly upon the number of Na ions, 

 less dissociation is suffered by each salt than would be the case if the 

 other were absent. 



It is found that nearly all salts, but only strong acids and strong 

 bases, suffer a large amount of dissociation in dilute solution. Weak 

 acids and bases undergo little or no ionisation, and it is upon this 

 fact that the weakness of the acid depends, the activity of an acid 

 being really measured by the proportion of ionised hydrogen in its 

 solution. 



It is thus evident that when two neutral salts are mixed in dilute 

 solution, if no insoluble or volatile product is formed by interaction of 

 their ions, no chemical action really takes place. Thus, if sodium 

 chloride and potassium nitrate be mixed together no evidence of 

 chemical action is presented ; indeed, none occurs, and in the mixed 



+ + 

 solution the ions K, Na, Cl, and NO 3 remain side by side. 



If, however, two of the ions can unite and form a non-ionisable 

 substance, then a decided interaction, accompanied usually by a 

 thermal disturbance, occurs. Such a non-ionisable substance may be 

 an insoluble body or it may be a soluble substance, e.g., water, which 

 is practically non-dissociated. 



