464 SMITH'S INTERMEDIATE CHEMISTRY 



analysis we discriminate between salts soluble in water, those 

 soluble in acetic acid (the insoluble carbonates and some sulphides, 

 FeS and MnS, for example), those requiring active mineral acids 

 for their solution (calcium phosphate, zinc sulphide and the more 

 insoluble sulphides, for example), and those insoluble in all acids 

 (barium sulphate and other insoluble salts of active acids). 



Precipitation of Insoluble Salts in Presence of Acids. 



The converse of solution, namely, precipitation, depends upon the 

 same conditions: an insoluble salt which is dissolved by a given 

 acid cannot be formed by precipitation in the presence of this 

 acid. Thus zinc sulphide can be precipitated in presence of acetic 

 acid, but not in presence of active mineral acids in ordinary con- 

 centrations. Cupric sulphide or barium sulphate can be precipi- 

 tated in presence of any acid, but ferrous sulphide and calcium 

 carbonate only in the absence of acids. 



From this it does not follow that zinc sulphide, for example, 

 cannot be precipitated if once an active acid has been added to the 

 mixture. To secure precipitation, all that is necessary is to 

 remove the excess of hydrogen-ion which is repressing the ioniza- 

 tion of the hydrosulphuric acid. This can be done by adding a 

 base, or by adding ammonium sulphide. 



Exercises. 1. (a) How do the electrolytic methods of mak- 

 ing calcium and magnesium differ? (b) Why not electrolyze an 

 aqueous solution of magnesium chloride in making magnesium? 

 (c) Why use both potassium and sodium chlorides in making mag- 

 nesium? (d) Why is magnesium, but not potassium or sodium, 

 liberated? 



2. Why are magnesium and zinc not found free in nature? 



3. Why does magnesium rust completely (in time), while zinc 

 does not? 



4. Make equations for: (a) the action of magnesium on hydro- 

 chloric acid; (b) the burning of magnesium in air; (c) the heating 





