310 SMITH'S INTERMEDIATE CHEMISTRY 



hydrogen peroxide does not act, so that it is a more active oxi- 

 dizing agent than is that substance : 



HNO 3 + 3HC1 -> NOC1 + 2H 2 + C1 2 . 



The mixture of concentrated hydrochloric acid, nitric acid, and 

 water is called aqua regia, and has strong oxidizing properties, due 

 to the presence of hypochlorous acid (from C1 2 + H 2 0) as well as 

 nitric acid. 



Action of Nitric Acid on Metals. Magnesium, and metals 

 above it in the activity list, will displace hydrogen freely, especially 

 from diluted nitric acid: 



Mg + 2HN0 3 - Mg(N0 3 ) 2 + H, T . 



But, with the less active metals, oxidation takes place, and instead 

 of hydrogen, we get water and, of course, a reduction product of 

 the nitric acid. The nitrate of the metal, however, is formed also. 

 Even metals, like copper and silver, which do not displace hydro- 

 gen, are acted upon by nitric acid in the same way (compare 

 sulphuric acid, p. 271). For example, diluted nitric acid acts 

 vigorously upon copper, giving nitric oxide NO as the reduction 

 product : 



Skeleton: Cu + HNO 3 - Cu(NO 3 ) 2 + NO + H 2 O. 

 Balanced: 3Cu + 8HNO 3 -> 3Cu(N0 3 ) 2 + 4H 2 O + 2NO. 



The nitric oxide is a colorless gas, but unites with oxygen in the 

 air to give NO 2 . 



A test for a nitrate may be founded on this action. To the 

 nitrate sulphuric acid is added to liberate nitric acid. Then 

 copper turnings are thrown in to give NO. A gas turning red as 

 it meets the air shows that a nitrate was present. 



Action Upon Organic Compounds. Nitric acid stains the 

 skin and nails yellow, by giving colored compounds with the pro- / 

 teins. It gives similar compounds with wool (a protein), and 



