NITRIC ACID 311 



therefore produces, on clothing, yellow stains which cannot be 

 removed. 



The explosives made by the use of nitric acid are discussed in 

 Chapter XL. . 



JVitrtc Oxide NO. This oxide is made by the action of diluted 

 nitric acid upon copper, by the action already discussed (p. 310). 

 It is ^colorless fias. and almost insolublejm. water. 

 Vigorously burning phosphorus continues to burn in it: 



4P + 10NO -> 2P 2 O 5 + 5N 2 . 



Its most important property is that rrf unita.^ m/A ^/p^ f f ft 

 give nitrogen tetrgxids^ 



2 ->2N0 2 . 



reaction is reversible at high temperatures. 



Nitrogen Tetroxide NO2. This oxide is formed by the. union 

 of oxygen with nitric oxide. It is given off, also, when concen- 

 trated nitric acid acts upon metals and other reducing substances. 

 It is further produced, along with oxygen, when nitrates, excepting 

 those of potassium, sodium and ammonium are heated, dry: 

 2Cu(N0 3 ) 2 -> 2CuO + 4N0 2 + O 2 . 



Potassium and sodium nitrates, when heated, give off only 

 oxygen, and leave the nitrites : 



2KN0 3 ->0 2 + 2KN0 2 . 



/ Nitrogen tetroxide is a red gas, with a choking odor. When 



J heated strongly, it becomes colorless, being dissociated into NO 



< and oxygen. When cooled, it becomes pale yellow, and its den- 



I sity becomes twice as great. This is due to the formation of mole- 



l cules of the formula N 2 4 : 



