THEORY OF ELECTROLYTIC DISSOCIATION, ETC. 193 



molecules are dissociated. Conversely, if the concentration of one 

 kind of ion is increased, as by the addition of a substance giving the 

 same kind of ion for example, addition of hydrochloric acid to a 



solution of sodium chloride (both having a common ion, Cl') the 



result is a reunion of some of the ions of the original substance to 

 form molecules, until the balance between its molecules and all of the 

 ions of the kinds that it gives satisfies the demands of the ionization 

 constant. If the solution is saturated in the beginning, and there- 

 fore contains all the undissociated molecules that it can hold, the for- 

 mation of an additional number of molecules by union of ions gives 

 rise to supersaturation and precipitation of some of the substance. 

 Thus, the addition of hydrochloric acid gas to a saturated solution of 

 common salt causes a copious precipitate of the salt. Likewise, addi- 

 tion of some saturated solution of the very soluble sodium chlorate 

 to one of the less soluble potassium chlorate causes precipitation of 

 some of the latter salt. The reason the sodium chlorate is not also 

 reciprocally precipitated is that the additional molecules formed by 

 union of its ions, Na" and CIO/, are not sufficient to supersaturate the 

 volume of liquid through which the sodium chlorate is distributed. 

 The principle just discussed furnishes an explanation of the fact that 

 is often observed in practical work ; namely, that many substances 

 are much less soluble in solutions of other substances of similar com- 

 position than in pure water. 



In the case of a solution of a slightly dissociating substance, the 

 addition of another substance having a common ion with the first 

 may so far cause a reversal of dissociation of the first substance that 

 practically only its undissociated molecules exist in the solution, with 

 an accompanying loss of some of its properties. Thus, in a solution 

 containing per liter the molecular weight of sodium acetate and the 

 molecular weight of acetic acid, the latter no longer is able to affect 

 the indicator methyl-orange, because there are too few hydrogen ions 

 of the acid left in solution. 



Precipitation. When molecules of a substance are dissolved, some 

 dissociate until a balance is established between ions and molecules, as 

 represented by the ionization constant. Conversely, when there are 

 present in a solution two substances which between them produce ions 

 corresponding to a third substance, some molecules of the third sub- 

 stance are formed up to the point that corresponds to its ionization 

 constant. The following ionic equations for a mixture of potassium 

 chloride and sodium nitrate in solution will illustrate : 



13 



