458 



Precipitation by Double Decomposition. The mechanism 

 of this type of reaction has already been discussed in some detail 

 in an earlier chapter (p. 185), to which reference should here be 

 made by the reader. The solubility-product law, however, throws 

 much additional light on the subject, and enables us to forecast 

 the completeness of any given precipitation under given conditions. 



The first thing to be remembered is that the precipitate which 

 we observe, however insoluble its material may be, does not 

 include all of the substance, but only the excess beyond what is 

 required to saturate the water. The liquid surrounding the pre- 

 cipitate is always a saturated solution of the substance precipitated. 

 If it were not so, some of the precipitate would dissolve until the 

 liquid became saturated. Thus, for example, when we add ammo- 

 nium sulphide solution to zinc chloride solution (p. 451) : 



2Cl- + Zn++ 



tl tl 



2NH 4 C1 ZnS ? ZnS 

 (dissolved) (solid) 



the liquid is a saturated solution of zinc sulphide, with the excess 

 of this salt suspended in it as a precipitate. 



Looking at the matter from this viewpoint, we perceive the 

 application of the rule of solubility-product constancy. In this 

 saturated solution the product of the ion-concentrations, [Zn++] X 

 [S = ], is constant. If the original solutions had been so very 

 dilute that, when they were mixed, the product of the concentra- 

 tions of these two ions had not reached the value of this constant, 

 no precipitation would have occurred. As a matter of fact, the 

 original salts are so extensively ionized in solution, and the solu- 

 bility-product of zinc sulphide is so small, that in all ordinary 

 mixtures the product [Zn++] X [S = ] considerably exceeds the 

 requisite value, and hence the salt is thrown down until the bal- 

 ance remaining gives the value in question. The rule for pre- 

 cipitation, then, is as follows: Whenever the product of the con- 



