Oxygen 261 



sodium oxide and the black oxide of iron as examples. 

 Should the basic oxide, when combined with the 

 elements of water, form a hydroxide soluble in water, 

 this solution is alkaline in reaction. Those hydroxides 

 insoluble in water, such as ferric hydroxide, the sub- 

 stance formed when the brown oxide the chief con- 

 stituent of iron rust is caused to combine indirectly 

 with the elements of water, cannot, of course, show any 

 action upon litmus, and hence such hydroxides are 

 not alkalis. They are termed simply bases. Most of 

 the metallic hydroxides are of this class. We shall 

 return to this method of classification of the oxides later. 



Water itself hydrogen oxide is neutral in reaction, 

 and is hence known as a neutral oxide. Further, when 

 two or more oxides of a metal are known, and the 

 highest gives off oxygen under suitable conditions, 

 this oxide is termed a peroxide (Latin, per, very). 



179. Several examples of chemical reactions have 

 now come under our notice, and we have had evidence 

 that a considerable amount of energy must be contained 

 within the substances taking part in the various changes 

 studied. This form of energy is called " chemical " 

 energy. During many of the actions chemical energy 

 has been degraded into heat, e.g. large quantities of 

 heat were produced during the combustions of the 

 different substances burnt in oxygen. On the other 

 hand, heat had to be supplied in order to maintain 

 some of the changes, such as those in which mercuric 

 oxide was made from its elements, and vice versa. 

 Here heat energy is changed into chemical energy. 

 Reactions of the first class, in which heat is evolved, are 

 termed "exothermic", while those in which the con- 

 tinuous provision of heat is required for the completion 

 of the change are called " endothermic " reactions. 



