MASS ACTION. 85 



Or again H + C1 + Na + OH = Na + Cl + H a O 



Ions of hydro- Ions of caustic Ions still in Water. 



chloric acid. soda. solution. Non-ionised. 



Or N + a + C a H 8 O a + H+NO. - Na + NO 8 + 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 8 2) as acetic acid is slightly dissociated. In- 



deed, in every case very small quantities of the ions of the 

 assumed non-ionisable substance remain in solution, as pro- 

 bably 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 tirne, but if other 

 changes occur, very often they do so slowly and the reaction 

 extends over a considerable period. 



Mass Action. When two substances are brought into con- 

 tact 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 carbonate (also insoluble) and potas- 

 sium sulphate, in accordance with the equation 

 K a CO, + BaSO 4 = BaCO 8 + K 2 S0 4 . 



But this reaction never goes on to completion as is repre- 

 sented by the equation. When equilibrium is attained -the 

 solution is found to contain both potassium sulphate and car- 

 bonate 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 



