xix BALANCED ACTIONS 513 



ammonium chloride which is not strong enough to precipitate 

 the divalent hydroxides of the metals (id), thus : 



A1C1 3 + 3 NH 4 OH -> Al(pH) s 



(precipitated) 



but ZnCl 2 + 2NH 4 OH <- Zn(OH) 2 + 2NH 4 Cl. 



(dissolves) 



In class (3) a similar method is used to separate magnesium 

 from calcium, strontium, and barium. Like chalk and barium 

 carbonate, magnesium carbonate is insoluble in water ; it is, 

 however, soluble in presence of ammonium salts, thus : 



2Na 2 CO 3 -* MgC0 3 + 2NaCl 



(precipitated) 1 



but MgCl 2 + 2(NH 4 ) 2 CO 3 <- MgC0 3 +2NH 4 Cl. 



(dissolves) 



In the former case, the tendency of the strong acid (HC1) to 

 combine with the stronger base (NaOH) drives the action in the 

 direction L to R, already indicated by the insolubility of 

 magnesium carbonate. In the latter case the magnesia is the 

 stronger base and the action from R to L is assisted further (i) 

 by the mass-action of an excess of ammonium chloride and (ii) 

 by the tendency of this substance to unite with magnesium salts 

 to form compounds, such as MgCl 2 , NH 4 C1,6H 2 O, and thereby 

 to weaken the mass-action of the magnesium salt. 



1 Usually as a basic salt. 



L L 



