66 CHEMISTRY. 



does not remain an oxide. It is immediately laid bold of 

 by some of the acid, and they together make a substance 

 called nitrate of copper and water. And so the process 

 goes on, some of the particles of nitric acid constantly giv- 

 ing oxygen to the copper, and other particles as constantly 

 seizing upon the oxide of copper thus formed, till the cop- 

 per is all changed to nitrate of copper. This lengthy ex- 

 planation is conveniently abridged in the following equa- 

 tions : 



Copper. Nitric acid. Oxide of copper. Nitric oxide. Water. 

 3Cu + 2HNO 3 = 3CuO + 2NO + H 2 O 



Oxide of copper. Nitric acid. Nitrate of copper. Water. 

 3CuO + 6HNO 3 = 3(Cu(N0 3 ) 2 ) + 3H 2 O 



The hydrogen of the acid takes to itself oxygen and 

 forms water in each case. It is this action of nitric acid 

 upon the oxides of metals which makes it so useful in 

 cleansing the surface of instruments or vessels made of 

 metals, as brass and copper, when they have become oxi- 

 dized from exposure or any other cause. The acid dissolves 

 the oxide, forming a salt with it, and thus makes the sur- 

 face bright. 



74. Exceptions. Most of the metals are acted upon in 

 the same way by nitric acid. The action upon tin and an- 

 timony is different from that which we have described in the 

 case of copper. Only one step of the process is taken with 

 these metals. The nitric acid merely parts with a portion 

 of its oxygen, and forms oxides of these metals. No solu- 

 tion is made, but we have the oxides in the form of a white 

 powder. Gold and platinum are not acted upon at all by 

 this acid. The reason is that they are not oxidizable, and 

 so the acid keeps all its oxygen to itself. 



75. Nitrates. These bodies are formed either by direct 

 union of nitric acid with the oxides or by the action of the 



