132 



Double Decomposition. In this chapter we have met for 

 the first time with another variety of chemical change. If we ex- 

 amine the equation for the action of silver nitrate on hydrochloric 

 acid (p. 131), we shall see that the silver nitrate decomposed as if 

 it had been made up of two parts, namely (Ag) and (N0 3 ). The 

 hydrochloric acid similarly separated into its two parts (H) and 

 (Cl). The (Ag) then united with the (Cl) and the (H) with the 

 (N0 3 ). 



(Ag)(N0 3 ) + (H)(C1) - (Ag)(Cl) + (H)(N0 3 ). 



Since both original substances decomposed, this whole change 

 is called a double decomposition. A sort of exchange between 

 the halves of the decomposing substances took place. 



The hydrogen chloride was prepared by an action (p. 126), 

 which, if we write it as follows, is seen to be of the same class : 



(Na)(Cl) + (H)(HS0 4 ) -> (H)(C1) + (Na)(HSO 4 ). 



The Varieties of Chemical Change. Almost all chemical 

 changes belong to one or other of the varieties we have already 

 met with and defined (pp. 14, 16, 51). These, along with one 

 example of each, are now placed together: 



1. Combination: Zn -f S -> ZnS. 



2. Decomposition: 2KC1O 3 -> 2KC1 + 30 2 . 



3. Displacement: Zn + H 2 S0 4 -> H 2 -f- ZnSO 4 . 



4. Double Decomposition: AgN0 3 + HC1 -> AgCl -f HN0 3 . 



In the first, 2 substances give 1 substance. 



In the second, 1 substance gives 2 (or more) substances. 



In the third, 1 element and 1 compound give 1 element and 1 com- 



pound. 

 In the fourth, 2 compounds give 2 compounds. 



This classification suffices for most purposes. But, for special 

 kinds of cases, some other names are used. Thus, a dissociation 



