MAGNESIUM AND ZINC. IONIC EQUILIBRIA 461 



This particular action is a neutralization of an insoluble base. 

 But the other kinds of actions by which insoluble electrolytes pass 

 into solution all resemble it closely, and differ only in details. The 

 general outlines of the explanation are the same in every case. 

 We proceed now to apply it to the common phenomenon of the 

 solution of an insoluble salt by an acid. 



Interaction of Insoluble Salts with Acids, Resulting in 

 Solution of the Salt. Zinc sulphide passes into solution 

 when in contact with acids, especially active acids. Thus, with 

 hydrochloric acid, it gives zinc chloride and hydrosulphuric acid, 

 both of which are soluble: 



ZnS t + 2HC1 + ZnCl 2 + H 2 S. (1) 



The action of acids upon insoluble salts is so frequently mentioned 

 in chemistry and is so important a factor in analytical operations 

 that it demands separate discussion. This example is a typical 

 one and may be used as an illustration. 



According to the rules already explained (p. 457), zinc sulphide 

 (or any other salt) is precipitated when the numerical value 

 of the product of the concentrations of the two requisite ions 

 [Zn++] X [S = ] exceeds the value of the ion-product for a saturated 

 solution of zinc sulphide in pure water. When, on the contrary, 

 the product of the concentrations of the two ions falls below the 

 limiting value, a condition which may arise from the removal in 

 some way either of the Zn ++ or of the S = ions, the solid will 

 dissolve to replace them until the ionic concentrations necessary 

 for equilibrium with molecules have been restored or until the 

 whole of the solid present is consumed. Here the sulphide-ion 

 from the zinc sulphide combines with the hydrogen-ion of the 

 acid (usually an active one) which has been added, 'and forms 

 molecular H 2 S : 



S" + 2H+ fc? H 2 S. (2) 



