MAGNESIUM AND ZINC. IONIC EQUILIBRIA 463 



Precipitates Insoluble in Acids. But even active acids 

 frequently fail to bring salts of weak acids into solution. Here 

 the cause lies in the fact that such salts are even less soluble 

 than those of the zinc sulphide type. Thus, even hydrochloric 

 acid (normal) will not appreciably dissolve cupric sulphide. The 

 solubility product [Cu 4 " 1 "] X [S = ] for this salt is so small that, after 

 an infinitesimal amount has gone into solution, the sulphide-ion 

 concentration is sufficient, in spite of the repressive action of the 

 large hydrogen-ion concentration furnished by the HC1, to bring 

 the product [Cu + +] X [S = ] up to its maximum possible value. 

 In this case the first link in the chain of equilibria: 



CuS (solid) 3 CuS (dissolved) *= Cu++ + S= 

 2HC1 +2Cl- + 2H+ 



u u 



CuCl 2 H 2 S <= H 2 S 



(dissolved) (gas) 



tends so decidedly backward that only the use of very concen- 

 trated acid will increase the concentration of the H+ to an extent 

 sufficient to secure even a slight advance of the whole action. 

 We must add, therefore, to the above rule: provided also that the 

 salt is not one of extreme insolubility. 



Illustrations of the application of these generalizations are 

 countless. Carbonic acid is made from marble (p. 333), hydrogen 

 sulphide from ferrous sulphide (p. 253), hydrogen peroxide from 

 sodium peroxide (p. 221), and phosphoric acid from calcium phos- 

 phate (p. 412). In each case the acid employed to decompose the 

 salt is more active than the acid to- be liberated. On the other 

 hand, calcium phosphate is insoluble in acetic acid because this 

 acid is weaker than is phosphoric acid. We have thus only to 

 examine the list of acids showing their degrees of ionization (p. 

 189) in order to be able to tell which salts, if insoluble in water, 

 will be dissolved by acids and, in general, what acids will be 

 sufficiently active in each case for the purpose. In chemical 



