186 SMITH'S INTERMEDIATE CHEMISTRY 



all double decompositions.) The mixture contains, then, no 

 fewer than eight different solutes, four ionic and four molecular, 

 and four reversible reactions control the equilibrium relation- 

 ships between them. 



Now it so happens that one of the new solutes, AgCl, is prac- 

 tically insoluble in water. Unless the solutions are excessively 

 dilute, therefore, silver chloride separates out from the solution 

 as a precipitate. This precipitation disturbs the existing equilibria 

 by withdrawing practically all silver ions Ag + and chloride ions 

 Cl~ from the solution. Undissociated AgN0 3 and HC1 continue 

 to break up, in an attempt to re-establish the equilibria, until 

 they also are practically eliminated, and only hydrogen ions H+ 

 and nitrate ions N0 3 ~, in equilibrium with undissociated HN0 3 , 

 are left in quantity in the solution. 



The reader may feel, at this point, that the previous method of 

 writing the reaction (p. 131) : 



AgN0 3 + HC1 -> AgCl j + HN0 3 



expresses all this more simply, without the need of worrying about 

 ions. But he must note that any silver salt added to any chloride 

 in solution also gives a precipitate of silver chloride. This can 

 be predicted immediately by ionic equations; any two solutions 

 containing Ag+ and Cl~ respectively must give a precipitate of 

 AgCl when mixed. Without the use of ionic equations, however, 

 we should have no guide to the course of any such reaction; 

 we should have to try it out for every mixture, memorize what 

 happened in each case, and then attempt to draw up some em- 

 pirical rules to assist us in retaining our tremendous mass of 

 isolated facts. The use of the ionic hypothesis obviates all this. 

 We do not require to remember all the properties of hundreds of 

 different acids, bases and salts in solution singly and mixed. 

 We know that each one of these substances exhibits two sets of 

 properties in solution, and that one set can be referred to the 

 positive ion, one set to the negative ion. If, therefore, we learn 



