OXIDIZING SUBSTANCES 227 



The transfer of combined oxygen from one substance to another 

 in a reaction is again oxidation : 



2KMnO 4 + 16HC1 -> 8H 2 O + 2KC1 + 2MnCl 2 + 5C1 2 . 

 Mn0 2 + 4HC1 - MnCl 2 + C1 2 + 2H 2 O. 



What is the substance oxidized in the last three reactions? It 

 is hydrochloric acid, and the product of its oxidation is chlorine. 

 True, we have added no oxygen to chlorine itself in any of these 

 reactions, but we have done something which is exactly equivalent 

 to addition of oxygen, we have taken away hydrogen, and we have 

 not replaced that hydrogen by any other positive element. 



Note that every oxidation is accompanied by reduction of the 

 oxidizing agent. Thus in the first of the three reactions just dis- 

 cussed, the free oxygen is reduced to water. In the other reactions, 

 KMnC>4 and MnO 2 are reduced to MnCl2. In all three cases 

 HC1 is the reducing agent. 



The appearance of a product that could only be formed by 

 reduction is sometimes the first thing that calls our attention to 

 the fact that an oxidizing action has occurred. When concen- 

 trated sulphuric acid acts upon hydrogen iodide (p. 204), the 

 iodine vapor given off on warming shows that there was oxidation, 

 but the odor of the hydrogen sulphide is the first thing we notice 

 when doing the experiment : 



H 2 S0 4 + SHI - H 2 S + 4H 2 + 4I 2 . 



Removal of the elements of water from a compound is neither 

 oxidation nor reduction, for hydrogen and oxygen are both removed: 



H 2 CO 3 -CO 2 + H 2 0. 

 NH 4 OH-*NH 3 + H 2 0. 



A I We can now see tha.t. rm'Hfl.tinn, in the, ahrnw mart*, nnnsiats n.lwa.yp_ 

 in adding oxygen or removing. hydrogen. 



Other Cases of Oxidation. But oxygen is only one of a class 

 of elements which we call non-metallic or negative elements, so 

 that we do not restrict the term " oxidation " to actions involving 



