90 CHEMISTRY FOR AGRICULTURAL STUDENTS 



Probably all these gases will be present, but when the nitric acid is strong 

 the higher oxides will predominate; when weak the lower oxides and 

 nitrogen will predominate. The action of nitric acid on most metals 

 results in the formation of a nitrate of the metal. Thus if copper be dis- 

 solved in nitric acid the nitric acid is reduced, and the oxide of copper 

 formed dissolves in more of the nitric acid, forming a blue solution of 

 copper nitrate. From this solution crystals of the salt may be obtained by 

 evaporation. The reason why no nitrate is formed in the case of tin is that 

 the higher oxide of tin is produced, and this, like some other higher 

 metallic oxides, is an acidic, not a basic, oxide (see p. 64). It will have been 

 noticed that in the preparation of nitric acid reddish coloured fumes of 

 nitrogen peroxide were formed. This is due to the dissociation of the 

 vapour of nitric acid by heat into nitrogen peroxide, water, and oxygen. 

 Light has also the same effect, and a bottle of nitric acid exposed to sun- 

 light soon becomes reddish in colour. 



If during the action of the zinc on the nitric acid the liquid 

 be kept very cold, it will soon acquire a green colour. The 

 colour is due to the presence of nitrous acid, a very unstable 

 compound soon decomposing into oxides of nitrogen and 

 water. This acid combines with ammonia, forming nitrite of 

 ammonium, but the salt is very unstable, for it decomposes 

 even at the ordinary temperature into nitrogen and water as 

 follows : — 



NH4N02 = N2 + 2H20. 



This reaction provides a simple method for preparing nitrogen 

 gas. Dissolve in water 25 grams of the nitrite of sodium, and 

 place the solution in a flask connected with two wash-botdes, 

 the first containing caustic soda solution, the second oil of 

 vitriol. To the contents of the flask add ammonium chloride 

 in the proportion represented by the formulae NaNOg + NH^Cl, 

 and warm gently. The mixture of salts will behave like nitrite 

 of ammonium, and nitrogen will be evolved. Note that, when 

 once the temperature is reached at which reaction commences, 

 no further heat is required. Try to explain this. Pass the 

 nitrogen into the density flask, and determine the relative 

 density by the method described on p. 39. 



Many other substances act as reducing agents on nitric acid. 



