64 



CIIEMISTKY. 



will understand them best by taking up nitric anhydride 

 first, and then following the others in the order given above. 



70. Nitric Anhydride, N 2 O 5 . This body is a great curios- 

 ity even to a chemist ; it is difficult to obtain, and hard to 

 keep when prepared ; and, since it has no good uses, you do 

 not care to learn much about it. When, however, we have 

 this substance united with water, we get the very impor- 

 tant acid known as nitric acid. We will give you the 

 reaction, although nitric acid is never prepared in this 

 manner, as you will presently see : 



N 2 5 +H 2 0=2(HNO 3 ). 



One molecule of nitric anhydride unites with one of wa- 

 ter, forming two molecules of nitric acid. 



71. Preparation of Nitric Acid. Potassium nitrate, com- 

 monly called either nitre or saltpetre, and sodium nitrate, are 

 the chief sources of nitric acid ; they are natural products, 

 but may also be made artificially, as you will learn hereafter. 

 Nitrate of sodium, heated with sulphuric acid also called 

 oil of vitriol gives us nitric acid and sodium sulphate : 



Sodium nitrate. 

 2NaNO 3 



Sulphuric acid. 

 H 2 SO 4 



Sodium sulphate. 



Nitric acid. 

 2HNO 3 



The process is 

 represented in Fig. 

 12. In the retort, 

 A, are the saltpetre 

 and the sulphuric 

 acid. The heat ap- 

 plied serves the 

 double purpose of 

 facilitating the 

 chemical change, 

 and of driving the 

 nitric acid as it is generated over into the receiver, B, in 



Fig. 12. 



