COMPOUNDS () l' MT];>(iI-:X WITH HVDRocKN AND OXYGEN 271 



metals the result of the reaction is usually not an oxide of the metal, 

 but the corresponding salt of nitric acid, and, at the same time, one of 

 the lower oxides of nitrogen. The resulting salts of the metals are 

 soluble, and hence it is said that nitric acid dissolves nearly all metals. 40 

 This case is termed the solution of metals by acids, although it is not a 

 case of simple solution, but a complex chemical change of the substances 

 taken. When treated with this acid, those metals whose oxides do not 

 combine with nitric acid yield the oxide itself, and not a salt ; for 

 example, tin acts in this manner on nitric acid, forming a hydrous 

 oxide, SnH 2 O 3 , which is obtained in the form of a white powder, 

 Sn + 4NH0 3 =H,SnO 3 + 4NO 2 + H 2 O. Silver is able to take up still 

 more oxygen, and to convert a large portion of nitric acid into nitrous 

 anhydride, 4Ag + 6HNO 3 =4AgNO 3 + N 2 O ; , + 3H 2 O. Copper takes 

 up still more oxygen from nitric acid, converting it into nitric oxide, 

 a perfectly colourless gas ; and, by the action of zinc, nitric acid 

 is able to give up a still further quantity of nitrogen, forming nitrous 

 oxide, 4Zn + 10NHO 3 =4Zn(NO 3 ). 2 + N 2 O + 5H 2 O. 41 Sometimes, and 

 especially with dilute solutions of nitric acid, the deoxidation pro- 

 ceeds as far as the formation of hydroxylamine and ammonia, and 



40 The application of this acid for etching copper or steel in engraving is based on 

 this fact. The copper is covered with a coating of wax, resin, &c. (etching ground), on which 

 nitric acid does not act, and then the ground is removed in certain parts with a needle, and 

 the whole is washed in nitric acid. The parts covered with the ground remain untouched, 

 whilst the uncovered portions are eaten into by the acid. Copper plates for etchings, 

 aquatints, &c., are prepared in this manner. 



il The formation of such complex equations as the above often presents some diffi- 

 culty to the beginner. It should be observed that if the reacting and resultant sub- 

 stances be known, it is easy to form an equation for the reaction. Thus, if we wish to 

 form an equation expressing the reaction that nitric acid acting on zinc gives nitrous 

 oxide, NoO, and zinc nitrate, Zn(NO 5 ).,, we must reason as follows : Nitric acid contains 

 hydrogen, whilst the salt and nitrous oxide do not ; hence water is formed, and therefore 

 it is as though anhydrous nitric acid, N.>O 5 , were acting. For its conversion into nitrous 

 oxide it parts with four equivalents of oxygen, and hence it is able to oxidise four equi- 

 valents of zinc and to convert it into zinc oxide, ZnO. These four equivalents of zinc 

 oxide require for their conversion into the salt four more equivalents of nitric anhydride, 

 consequently five equivalents in all of the latter are required, or ten equivalents of nitric 

 acid. Consequently ten equivalents of nitric acid are necessary for four equivalents of 

 /inc in order to express the reaction in whole equivalents. It must not be forgotten, 

 however, that there are very few such reactions which can be entirely expressed by simple 

 equations. The majority of equations of reactions only express the chief and ultimate 

 products of reaction, and thus none of the three preceding equations express all that 

 in reality occurs in the action of metals on nitric acid. In no one of them is only one 

 oxide of nitrogen formed, but always several together or consecutively one after the 

 other, according to the temperature and strength of the acid. And this is easily under- 

 stood. The resulting oxide is itself capable of influencing metals and of being deoxidised, 

 and in the presence of the nitric acid it may change the acid and be itself changed. The 

 equations given must be looked on as a systematic expression of the main aspects of re- 

 actions. Further, these reactions vary considerably with different temperatures and 

 varying strengths of acid. 



