xix BALANCED ACTIONS 511 



affinity of a series of different acids for a given base and 

 conversely. He distinguished between the relative attractions 

 "in the moist way," i.e. in solution and "in the dry way," i.e. 

 by ignition. Thus, we have : 



In the moist way : 



2NH 3 + PbCl 2 + 2H 2 O -> 2NH 4 Cl + Pb(OH) 2 . 

 In the dry way : 



2NH 4 Cl + Pb(OH) 2 -> 2NH 3 + PbCl 2 + 2H 2 O. 



Berthollet, in his "Researches into the Laws of. Chemical 

 Affinity" (1801), and in his " Chemical Statics" (1803), recognised 

 the influence of " mass-action," and suggested that " Every 

 substance which has a tendency to enter into combination, 

 acts in the ratio of its affinity and of its quantity." A weak 

 acid in large quantities may therefore compete with a stronger 

 acid in smaller quantities. The action of the weak acid " in the 

 wet way " is greatly helped if it forms an insoluble salt, e.g. : 



H 2 C 2 O 4 + 2CaCl 2 ^ CaC 2 O 4 +.2HCl. 



(insoluble) 



or "in the dry way" if it is less volatile than the stronger 

 acid, e.g. : 



H 2 C 2 O 4 + 2NaCl ^ Na 2 C 2 O 4 +2HCl. 



(volatile) 



The influence of mass is seen best, however, in the balanced 

 actions between pairs of neutral salts : these differ so little in 

 stability that the interaction is controlled almost entirely by 

 mass-action and solubility. The following cases may be noticed : 



(a) K 2 S0 4 + Ca(N0 3 ) 2 ^ CaS0 4 + 2KNO 3 , 



CaSO 4 is almost insoluble and is always precipitated first. 



() K 2 S0 4 +2NaNO 3 :^ Na 2 SO 4 + 2KN0 3 



K 2 SO 4 separates first on evaporation, but as NaNO 3 accumu- 

 lates in the solution the action passes from left to right and 

 KNO 3 crystallises. 



(c) 2KCl + Ca(NO 3 ) 2 ^ 2KN0 3 + CaCl 2 



KClor KNO 3 may crystallise according to the proportions in 

 which the salts are mixed. 



(d) NaNO 3 +KCl ^ KN0 3 + NaCl. 



