IONIZATION 185 



ZnCl 2 -h H 2 1 may be written: 



Zn + 2H+ + 2C1- -> Zn++ + 2C1~ + H 2 1 

 or: 



Zn-f 2H+->Zn++ + H 2 t. 



From this it appears (see however p. 195) that the action simply 

 consists of a transfer of positive charges from hydrogen ions to 

 atomic zinc, free hydrogen being liberated and zinc ion going into 

 solution. Similarly the action of zinc on a solution of a copper 

 salt may now be written: 



Zn 4- Cu++ -> Zn++ + Cu J. 



The activity series of the metals (p. 54) expresses, therefore, 

 the order of their preference for assuming the ionic state. 



Ions and Double Decomposition. The mechanism of reac- 

 tions of this type also becomes much clearer when we write the 

 equations in the ionic form. Thus, for the precipitation of silver 

 chloride by the action of hydrochloric acid on silver nitrate solu- 

 tion, we have : 



HC1 ;=* Cl- + H+ 



11 11 

 AgCl | HNO 3 . 



In the mixed solutions, we have four ions in quantity, Ag+ 

 and N0 3 ~ from the AgN0 3 , H+ and Cl~ from the HC1. Before 

 mixing, these were in equilibrium with undissociated AgNO 3 and 

 undissociated HC1 respectively. But as soon as we bring all 

 four ions into the same solution, we furnish them with the op- 

 portunity of combining with other partners, Ag + with Cl~ and H+ 

 with N0 3 ~. Undissociated AgCl and undissociated HN0 3 are 

 also present, therefore, in the mixed solutions, each substance in 

 equilibrium with its respective pair of ions. (This is conveniently 

 indicated by the method of writing the equations which is em- 

 ployed above. The student should copy this method in analyzing 



