176 NON-METALS AND THEIR COMBINATIONS. 



Nitric acid is a strong oxidizing agent ; this means that it is capable 

 of giving off part of its oxygen to substances having affinity for it. 



Nitric acid of 68 per cent, has a constant boiling-point and distils un- 

 changed. A more concentrated acid decomposes in part when distilled, 

 2HNO 3 = 2NO 2 4- H 2 + O, while a more dilute acid gives off water at first. 

 In either case, repeated distillation gives a 68 per cent. acid. 



The action of nitric acid upon such metals as copper, silver, and many others 

 involves two changes, viz. : displacement of the hydrogen of the acid by the 



metal : 



Cu + 2HNO 3 = Cu(NO 3 ) 2 + 2H; 



and the deoxidation of another portion of nitric acid by the liberated hydrogen 

 while yet in the nascent state. Thus : 



HNO 3 + 3H = 2H 2 O + NO. 



The liberated nitrogen dioxide, which is colorless, readily absorbs oxygen 

 from the air, forming red vapors of nitrogen tetroxide. 



Another explanation of the chemical action of nitric acid on metals is based 

 on our knowledge of the fact that nitric acid readily gives up part of its oxygen 

 to any substance having affinity for it. Therefore, it majjfce assumed that the 

 first action of the acid on metals 'is their conversion into oxides, which are 

 immediately changed into nitrates, thus: 



2HN0 3 + 3Cu = 3CuO + H 2 O + 2NO. 

 CuO + 2HNO,= Cu(NO 3 ) 2 + H 2 O. 



Tests for nitric acid or nitrates. 

 (Potassium nitrate, KNO 3 , may be used as a nitrate ) 



1. Fairly strong nitric acid with copper turnings gives copious red 

 fumes. Rather dilute acid, however, has not a very marked action, 

 even when heated, but the action is increased by the addition of some 

 concentrated sulphuric acid. A nitrate, dry or in solution, has no 

 action on copper, but addition of concentrated sulphuric acid (to set 

 free the nitric acid) causes evolution of red fumes. 



2. The solution of a nitrate, to which a few small pieces of ferrous 

 sulphate have been added, will show a reddish-purple or black color- 

 ation upon pouring a few drops of strong sulphuric acid down the 

 side of the test-tube, so that it may form a layer at the bottom of the 

 tube. The black color is due to the formation of an unstable com- 

 pound of the composition 2FeSO 4 .NO. Free nitric acid also re- 

 sponds to this test, which is delicate and very often used. 



3. Nitrates deflagrate when heated on charcoal by means of the 

 blow-pipe flame. The high temperature causes the nitrate to decom- 

 pose and liberate oxygen, which unites with the charcoal energetically. 



