112 PRINCIPLES OF CHEMISTRY. 



to take place cannot be foretold, but has to be studied experimentally, 

 there are other chemical changes which can be predicted with certainty. 

 This is more especially true in the case of the action of acids on 

 bases and the action of one salt on another salt. This will be easily 

 seen when the relationship between acids, bases, and salts is under- 

 stood. Among these classes of compounds the results can usually be 

 foretold, and there is little difficulty in representing the change by 

 the proper equation. In doing this it must be borne in mind that 

 equivalent quantities replace one another; that, for instance, two atoms of a 

 univalent element are required to replace one atom of a bivalent element, 

 as, for instance, in the case of the decomposition taking place between 

 potassium iodide and mercuric chloride, when two molecules of the 

 first are required to decompose one molecule of the second compound : 



K I rr /Cl TJ- /I K Cl 



K-I + Hg \Cl : Hg \I K-C1 

 or 



2KI + HgCl 2 = HgI 2 + 2KC1. 



Whenever the exchange of atoms takes place between univalent 

 and trivalent elements, three of the first are required for one of the 

 second, as in the case of the action of sodium hydroxide on bismuth 

 chloride : 



Na OH /Cl /OH Na Cl 



Na OH + Bi Cl == Bi OH + Na Cl 

 Na OH \C1 \OH Na Cl 



or 



3NaOH + BiCl 3 = Bi(OH) 3 + 3NaCl. 



In the following examples of double decomposition an exchange 

 takes place between the atoms of metallic elements, or between the 

 metallic elements and the hydrogen. The student, in completing the 

 equations, has also to select the correct quantity, i. e., the correct 

 number of molecules of the factors required for the change. The 

 interrogation marks indicate that more than one atom or one molecule 

 of the substance is needed for the reaction. 



Na' + H'Cl Cu"SO 4 + H/S = 



H/S0 4 -f K'(?) Ba"Cl 2 + Na/SO 4 



Ca" + H'Cl (?) = Na/C0 3 + H/SO 4 



Fe" + H/S0 4 = Bi"'(N0 3 ) 3 + K'OH (?) = 



H'Cl -f Ag'NOg = Al a /// (S0 4 ) 8 + K'OH (?) =r 



Ca"Cl 2 +Ag'N0 3 (?)= A1/"(S0 4 ) S + Ca"(OH) 2 (?) = 



Bi'^CL, + Ag'NOat?) = Fe 2 "'Cl 6 + Ag'NO 3 (?) = 



Types of chemical change. There are four principal ways in 

 which chemical actions take place. These may be represented by the 





