IV. THE KEACTIONS OCCUEKING IN SOILS 79 



For example Na + Cl + Ag + NO 3 = Na + NO 3 + AgCl 



Silver 

 chloride. 



Or again H + Cl + Na + OH = Na + Cl + H 2 



Ions of hydro- Ions of caustic Ions still in Water 



chloric acid. soda. solution. non-ionised. 



Or Na + C 2 H 3 O 2 + H + NO 3 = Na + NO, + HC 2 H 3 O 2 



Ions of sodium Ions of nitric Ions still free Acetic acid 



acetate. acid. in solution. very little ionised. 



+ 

 In the last case in the solution there would be a very few ions H 



and C 2 H 3 O 2 , as acetic acid is slightly dissociated. Indeed, in every 

 case very small quantities of the ions of the assumed non-ionisable 

 substance remain in solution, as probably no substance is absolutely 

 non-ionisable, just as probably no substance is absolutely insoluble in 

 water. 



When a chemical reaction takes place simply between ions it is 

 completed in an exceedingly short time, but if other changes occur, 

 very often they do so slowly and the reaction extends over a consider- 

 able period. 



Mass Action, When two substances are brought into contact by 

 one or both being in solution a reaction often occurs and goes on until 

 equilibrium is attained. 



This equilibrium is generally reached before the whole of a 

 reaction as represented by an equation is completed. What actually 

 occurs will perhaps be best understood by taking an example. If 

 barium sulphate (insoluble) be treated with a solution of potassium 

 carbonate a reaction begins, resulting in the formation of barium car- 

 bonate (also insoluble) and potassium sulphate, in accordance with 

 the equation 



K 2 C0 3 + BaSO 4 = BaC0 3 + K 2 SO 4 . 



But this reaction never goes on to completion as is represented by 

 the equation. When equilibrium is attained, the solution is found to 

 contain both potassium sulphate and carbonate and the precipitate 

 both barium carbonate and sulphate. Moreover, it is found that if 

 barium carbonate be treated with potassium sulphate solution, a 

 similar state of equilibrium is attained and the ratio between the 

 number of equivalents of potassium sulphate and potassium carbonate 

 present in the solution would be the same in each case (the ratio 

 actually found is about 1:4). The reactions really take place in both 

 directions at the same time, and equilibrium is attained when the 

 velocities of the two opposing reactions are equal. The reaction may 

 therefore be better represented thus : 



K 2 CO 3 + BaS0 4 1 K 2 SO 4 + BaCO 3 . 



