462 SMITH'S INTERMEDIATE CHEMISTRY 



It will be seen that the removal of the ions in this fashion can 

 result in considerable solution of the salt only when the acid pro- 

 duced is a feebly ionized one. Here, to be specific, the concentra- 

 tion of the S = in the hydrosulphuric acid equilibrium (2) above 

 must be less than that of the same ion in a saturated zinc sulphide 

 solution. Now hydrosulphuric acid belongs to the least active 

 class of acids, and its pure solution contains only a very small con- 

 centration of S = (p. 189). There is, however, another factor in the 

 situation which we have not yet taken into account. The hydro- 

 chloric acid which we used for dissolving the precipitate is a very 

 highly ionized acid and gives an enormously greater concentration 

 of hydrogen-ion than does hydrosulphuric acid. Hence the 

 hydrogen-ion is in large excess in equation (2), and the condition 



rjj+12 x rg=i 

 for equilibrium, r1J ai - = K, will be satisfied by a corre- 



spondingly much smaller concentration of S = . In this particular 

 case, therefore, the [S = ] of the hydrosulphuric acid is far less than 

 that given by the zinc sulphide. The whole change, therefore, 

 depends for its accomplishment, not only on the mere presence 

 of hydrogen-ion, but on the repression of the ionization of the hydro- 

 sulphuric acid by the great excess of hydrogen-ion furnished by 

 the active acid that has been used. The whole scheme of the equi- 

 libria is as follows : 



ZnS (solid) =? ZnS (dissolved) fc? Zn++ + S= 

 2HC1 fc2Cl- + 2H+ 



tl U 



ZnCl 2 H 2 S * H 2 S 

 (dissolved) (gas) 



A generalization may be based on these considerations: an 

 insoluble salt of a given acid will in general interact and dissolve 

 when treated with a solution containing another acid, provided 

 that the latter acid is a much more highly ionized (more active) 

 one than the former. 



