MAGNESIUM AND ZINC. IONIC EQUILIBRIA 457 



Illustration of the Principle of Solubility -Product Con- 

 stancy. When, to a saturated solution of one of the less soluble 

 salts, a concentrated solution of a very soluble salt having one 

 ion in common with the first salt is added, partial precipitation 

 of the first salt is found to take place. This happens, for example, 

 with a saturated solution of potassium chlorate, which is not very 

 soluble (molar solubility 0.52, see Table). The concentrations 

 [K + ] and [C1O 3 ~] being small, one may easily increase the value 

 for one of the ions, say [C10 3 ~], fivefold, by adding a chlorate 

 which is sufficiently soluble. To preserve the value of the product 

 [K+] X [C1O 3 -], the value of [K+] will then have to be diminished 

 at once to one-fifth of its former value. This can occur only 

 by union of the ionic material it represents with an equivalent 

 amount of that for which [C10 3 ~] stands. The molecular material 

 so produced will thus tend at first to swell the value of [KC10 3 ]. 

 But the value of [KC10 3 ] cannot be increased, for the solution 

 is already saturated with molecules, so that the new supply of mole- 

 cules, or others in equal numbers, will be precipitated. Hence 

 the ionic part of the dissolved substance may be diminished, the 

 equilibria (p. 455) may be partially reversed, and we may actually 

 precipitate a part of the dissolved material without introducing 

 any substance, which, in the ordinary sense, can interact with it. 



In point of fact, when, to a saturated solution of potassium 

 chlorate there is added a saturated solution of potassium chloride 

 KC1 (molar solubility, 3.9) or of sodium chlorate NaC10 3 (molar 

 solubility, 6.4), a precipitate of potassium chlorate is thrown down. 



The precipitation of sodium chloride from a saturated solution, 

 by the introduction of gaseous hydrogen chloride (p. 127), is to be 

 explained in the same manner. The equilibria: 



NaCl (solid) <= NaCl (dslvd ) Na+ + Cl- 

 are reversed by the introduction of additional Cl~ from the very 

 soluble, and highly ionized HC1. 



