218 NON-METALS AND THEIR COMBINATIONS. 



comes saturated, that is, it takes up the maximum number of molecules and 

 ions that it can hold (which, of course, is not large), and these are in equi- 

 librium. If in any way the concentration of the negative (acid radical) ion is 

 diminished, more molecules dissociate to keep up the concentration of that 

 ion, which results in the dissolving of more molecules of the solid salt to keep 

 up saturation and equilibrium. If the negative ions of the salt are of an acid 

 that has a slight dissociating power, their concentration will be diminished 

 when an active (highly dissociating) acid is added to the mixture, thereby 

 furnishing an abundance of H' ions, with which the negative ions of the salt 

 unite to form undissociated molecules of the acid. If the concentration of the 

 negative ions of the salt is greater than that which can be maintained by the 

 corresponding acid, the salt will dissolve by the addition of an active acid, in 

 keeping with the principle of equilibrium as involved in the dissociation con- 

 stant (see page 192). Even an acid of a moderate degree of dissociation may 

 have its dissociation reversed to such an extent in the presence of an excess 

 of a highly dissociating acid, like hydrochloric, that it becomes equivalent to 

 a slightly dissociating acid, and does not maintain as great a concentration of 

 its negative ion as is maintained by its slightly soluble salts in water. This is 

 illustrated by the solution of calcium oxalate in excess of hydrochloric acid. 

 In the case of highly insoluble salts, like barium sulphate, the amount dis- 

 solved, and consequently the concentration of its ions, is so extremely small 

 that addition of active acids has very little effect in reducing the concentration 

 of the negative ion. Hence, extremely little of such a salt is dissolved by 

 acids. Such salts evidently can be precipitated in an acid medium, whereas 

 the salts that are dissolved by a given acid cannot be precipitated in the 

 presence of that acid. 



The points just discussed may be given a more concrete form by the consid- 

 eration of the sulphide of iron and of copper. When dilute hydrochloric acid 

 is added to iron sulphide, hydrogen sulphide is evolved. The ionic repre- 

 sentation of the act of solution is the following : 



FeS (slightly soluble) ^ Fe' + S" \ R s 

 2HC1 ^ 2Cr + 2H' / " 



The negative ion S" coming from the slight amount of FeS dissolved in water 

 is also the ion of H 2 S. Hydrogen sulphide dissociates to a less degree than 

 does FeS ; that is, the concentration of S x/ that can be maintained by H 2 S in 

 solution is less than that which can be maintained by FeS in solution. The 

 result is that some S x/ ions unite with H' ions of the hydrochloric acid to 

 form undissociated molecules of H a S, thus reducing the concentration of S /x 

 ion. More FeS dissolves to keep up the equilibrium. This is kept up until 

 all the FeS is dissolved, or until (if FeS is in excess) the HC1 is so much ex- 

 hausted that the little which remains is in equilibrium with the other products. 

 As the H,S accumulates, the solution becomes saturated and the excess escapes 

 as gas. 



In the case of copper sulphide, dilute hydrochloric acid has no action. The 

 ionic reactions would be : 



CuS (very slightly soluble) \ ; Cu' + S" \ _^ R ~ 

 2HC1 / i 2C1' + 2H' / *~ 2 



