228 SMITH'S INTERMEDIATE CHEMISTRY 



oxygen. Thus, forming a sulphide, by heating a metal with 

 sulphur, is oxidation also : 



Fe + S-FeS. 



Similarly, changing ferrous chloride FeCk to ferric chloride FeCl 3 

 is oxidation: 



//- 



In every compound one of the elements is relatively positive 

 and the other relatively negative. Iron is positive, sulphur and 

 chlorine are negative. 



O Oxidation, then, is introducing, or increasing the proportion of 

 the negative element, or removing, or reducing the proportion of 

 the positive element. Reduction is the converse. 



Oxidation and Valence. Combining a negative element 

 with a metal raises the active valence of the latter from zero to 

 some finite value. Metallic copper has no valence in use. In 

 CuCU the copper is employing the valence II. The copper has 

 been oxidized. Similarly, changing FeCl 2 to FeCl 3 increases the 

 active valence of the iron from II to III. Conversely, changing 

 HC1 to C1 2 alters the active valence of the chlorine from I to zero. 

 Hence, oxidation may be defined as increasing the active valence 

 of a positive element or decreasing that of a negative element. 

 Reduction is the converse. 



Oxidation and Electrons. Finally, since increasing the 

 valence of a negative atom means adding one or more electrons to 

 that atom, and increasing the positive valence of an atom means 

 removing one or more electrons, we reach the briefest definition by 

 saying: Oxidation is removing electrons and reduction is adding 

 electrons. 



Exercises. 1. Why does air containing ozone lose the latter 

 (by change into oxygen) quicker when warm than when cold? 



2. Mark the valences of the radicals in barium peroxide and 

 hypochlorous acid. 



