460 SMITH'S INTERMEDIATE CHEMISTRY 



Rule for Solution of Substances. The rule for solution of 



any electrolyte follows at once from the foregoing considerations, 

 and may be formulated by changing a few of the words in the rule 

 just given : Whenever the product of the concentrations of any 

 two ions in a mixture is less than the value of the ion-product 

 in a saturated solution of the compound formed by their union, 

 this compound, if present in the solid form, will be dissolved. 

 When applied to the simplest case, this rule means that a sub- 

 stance will dissolve in a liquid not yet saturated with it, but 

 will not dissolve in a liquid already saturated with the same 

 material. The main value of the rule lies, however, in its appli- 

 cation to the less simple, but equally common cases, such as 

 when an insoluble body is dissolved by interaction with another 

 electrolyte. 



Applications of the Rule for Solution to the Solution of 

 Insoluble Substances. So long as a substance remains in pure 

 water its solubility is fixed. Thus, with calcium hydroxide, the 

 system comes to equilibrium at 18 when 0.17 g. per 100 c.c. of 

 water (0.02 moles per liter) have gone into solution : 



Ca(OH) 2 (solid) <=* Ca(OH) 2 (dslvd.) 5 Ca++ + 2OH~. 



But if an additional reagent which can combine with either one of 

 the ions is added, the concentration of this ion at once becomes less, 

 the actual numerical value of the ion-product therefore begins to 

 diminish, and further solution must take place to restore its 

 value. Thus, if a little of an acid (giving H + ) be added to the 

 solution of calcium hydroxide, the union of OH~ and H + to form 

 water removes almost all the OH~ (see p. 368), and solution of the 

 hydroxide proceeds until the acid is used up. There are now 

 more Ca++ than OH~ ions present, but the ion-product reaches 

 the same value as before, and then the change ceases. If a further 

 supply of acid is added, the removal of OH~ to form H 2 begins 

 again. Hence, with excess of acid, the calcium hydroxide finally 

 all dissolves. 





