84 THE REACTIONS OCCURRING IN SOILS. IV. 



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 dis- 

 turbed. Suppose, for example, sodium sulphate were added 



to the common salt solution. The ions of the new salt are 



+ + 



Na, Na, and SO 4 , and the conditions of equilibrium are deter- 

 mined by the equation 



k (Na,SO 4 ) = Na x Na x SO 4 



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

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

 partly by the sodium sulphate. The consequence is that 

 inasmuch as the degree of dissociation 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 suffer a large amount of 

 dissociation in dilute solution, but that this is only the case 

 with strong acids and strong bases. 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, 



Ol, 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-ioni- 

 sable substance may be an insoluble body or it may be a 

 soluble substance, e.g., water, which is practically non-disso- 

 ciated. 



+ + + 



For example Na + Cl + Ag + NO s = Na+NO 8 + AgCl 



Silver 

 chloride. 



