HYDROGEN 57 



oxide of iron or of copper is heated in a tube in a stream of 

 hydrogen, water is produced and the metal remains: 



Magnetic oxide of iron + Hydrogen > Iron -f- Water. 



Iron Hydrogen 



Oxygen Oxygen 



Oxides of metals above iron in the " order of activity " (p. 54), 

 however, are very stable. Hydrogen is unable to remove the 

 oxygen from such oxides and leave the metal. 



Reduction. The removal of oxygen from a compound by its 

 union with some other substance is called reduction and the 

 substance (in the foregoing instance, hydrogen) is called a reduc- 

 ing agent. Carbon, in the form of coal or coke, is the agent of this 

 kind most commonly used in chemical industries. The term 

 reduction is applied to some other chemical actions, in which oxy- 

 gen is not concerned. In all cases, however, reduction is the 

 opposite of oxidation (p. 41). 



Exercises. 1. Name three varieties of chemical change (pp. 

 8, 16, 51) and explain the difference between them. 



2. What do you infer as to the composition of a substance 

 when it is named: (a) an oxide, (b) an hydroxide (p. 51)? 



3. What are the equivalent quantities of: (a) carbon and oxy- 

 gen (p. 35), (b) zinc and sulphur? 



4. What law shows that the ratio of chemically equivalent 

 quantities of any two substances must be constant? 



5. Name the metals which: (a) do not liberate hydrogen from 

 water or dilute acids, (b) are found free in nature. 



6. Why does hydrogen gas, when poured out, flow upwards? 

 Why is it an excellent gas for filling balloons? 



