NITROGEN. 89 



stances which deoxidize it. It is capable of combining directly with 

 one or more atoms of oxygen, thereby forming NO 2 , nitrogen tetroxide, 

 which is a gas of deep red color and poisonous properties. Nitrogen 

 trioxide is of no practical interest. 



Experiment 7. Pour about 1 c.c. of nitric acid upon a few fragments of 

 metallic copper, and apply heat. Notice that red fumes escape, which are 

 nitrogen tetroxide, and that a blue solution is formed which contains cupric 

 nitrate. See explanation of the change below. 



Hyponitrous acid, HNO, and nitrous acid, HNO 2 , have not been 

 isolated and are known in combination only. Nitrous acid and 

 nitrites occur, in small quantity, in air, and also in water, where 

 they are formed by decomposition of nitrogenous organic matter. 



Nitrites evolve red fumes on the addition of sulphuric acid ; they act as 

 reducing agents decolorizing acidified solution of potassium permanganate; 

 they color blue a mixture of zinc iodide and starch solution ; they give a dark- 

 brown color with solution of meta-phenylene-diamine, C 6 H 4 (NH 2 ) 2 , in the 

 presence of free sulphuric acid. 



Nitric acid, Acidum nitricum, HNO 3 63 (Aqua fortis, Hydric 

 nitrate). Nitrogen pentoxide, N 2 O 5 , a white, solid, unstable com- 

 pound, is of scientific interest only. When brought in contact with 

 water it readily combines with it, forming nitric acid : 



N 2 O 5 + H 2 O = 2HNO 3 . 



The usual method for obtaining nitric acid is the decomposition of 

 sodium nitrate by sulphuric acid : 



NaN0 3 4- H 2 SO 4 = HNO 3 + HNaSO,; 



Sodium 

 bisulphate. 



or 



2NaN0 3 + H 2 SO 4 == 2HNO 3 + Na 2 SO 4 . 



Sodium 

 sulphate. 



Experiment 8. Prepare an apparatus as shown in Fig. 9. Heat in a retort of 

 about 250 c.c. capacity a mixture of about 50 grammes of potassium nitrate 

 and nearly the same weight of sulphuric acid. Nitric acid is evolved and distils 

 over into the receiver, which is to be kept cool during the operation by pouring 

 cold water upon it or by surrounding it with pieces of ice. Examine the 

 properties of nitric acid thus made, and use it for the tests mentioned below. 

 How much pure nitric acid can be obtained from 50 grammes of potassium 

 nitrate ? Weigh the acid which you obtain in the experiment and compare 

 this weight with the theoretical quantity. 



The acid thus obtained is an almost colorless, fuming, corrosive 

 liquid, of a peculiar, somewhat suffocating odor, and a strongly acid 



