456 SMITH'S INTERMEDIATE CHEMISTRY 



If we apply to the two reversible reactions represented in this 

 scheme the principles which have been developed above, we obtain 

 the two equations: 



[KC10 3 (dissolved)] 

 [KC10 3 (solid)] 



[KC1O 3 (dissolved)] 



-K. (2) 



where Ki and K 2 are the equilibrium constants of the two reac- 

 tions. By multiplying equation (1) by equation (2), we obtain a 

 third relationship: 



[K + ] [CIO,-] 



[KC1O 3 (solid)] 



(3) 



Now [KC1O 3 (solid)], the concentration of KC10 3 in the solid state, 

 is invariable at any fixed temperature. Equation (3) conse- 

 quently reduces to the form : 



[K+] [ClOr] = KiK 2 [KClOa (solid)] = a constant quantity. 



In other words, in a saturated solution, the product of the molar 

 concentrations of the ions is a constant. This product is called the 

 solubility product of the substance. The law of the constancy 

 of the ion-product in a saturated solution is one of the most useful 

 of the principles of chemistry. It enables us to explain all the 

 varied phenomena of precipitation and of the solution of pre- 

 cipitates in a consistent manner, as will be seen below. Upon 

 the accuracy of the law of the constancy of the solubility product 

 our whole system of quantitative analysis is based. It is of 

 importance therefore to mention the fact that, although experi- 

 mental test has shown that both equation (1) and equation (2), 

 by means of which the law was derived, are grossly inaccurate for 

 all strong electrolytes (neither KI nor K 2 remaining by any means 

 constant as conditions are varied), yet equation (3), the law itself, 

 always approximates very closely to the truth. 



