494 SMITH'S INTERMEDIATE CHEMISTRY 



a white luster, is very tough, and melts at about 1510. The 

 purest iron does not rust in cold water. 



Ordinary iron rusts in moist air or under water, forming a 

 hydrated ferric oxide 3Fe 2 3 ,H 2 0. The impurities act as contact 

 agents. The rust is a brittle, porous, non-adherent coating, which 

 does not protect the metal below. Oil protects iron from rusting 

 because, although oxygen penetrates the oil, being soluble in it, 

 moisture does not. Iron displaces hydrogen from hydrochloric 

 and sulphuric acids, giving ferrous salts : 



Fe + 2H+ - Fe++ + H 2 1 . 



The impurities carbide, sulphide, and phosphide produce 

 hydrocarbons, hydrogen sulphide, and phosphine PH 3 , and the 

 last two confer an odor on the escaping gas. 



Iron burns in oxygen, and acts when heated upon steam, in both 

 cases producing magnetic oxide of iron Fe 3 O 4 (p. 51). A thin film 

 of this oxide is adherent, and protects the iron (" Russia " iron). 

 The articles to be treated are put into a closed retort and exposed 

 first to a current of superheated steam and then to a current of 

 producer gas (p. 337) to reduce any higher oxides that may have 

 been formed. Watch hands, buckles and the like may be given a 

 protective coating by dipping them in an oxidizing bath such as 

 melted saltpeter. Another method of rust proofing is to immerse 

 iron articles in a hot solution of ferrous phosphate. This salt is 

 appreciably hydrolyzed in solution, and the free acid acting upon 

 the iron converts its surfaces into an adherent film of basic phos- 

 phate. 



Iron has Two Valences. One atomic weight of iron may 

 hold two or three atomic weights of a univalent element in com- 

 bination. Thus, we have ferrous chloride FeCl 2 and ferric chlo- 

 ride FeCl 3 , the bromides FeBr 2 and FeBr 3 , the oxides FeO and 

 Fe 2 3 , ferrous sulphate FeS0 4 and ferric sulphate Fe 2 (S0 4 ) 3 , and 

 so forth. It jnav therefore be bivalent or trjvalent, according to 

 the chemical conditions. 



