496 SMITH'S INTERMEDIATE CHEMISTRY 



When Only a little _ryKnnn TiyHmviHp is arMpH, f.hp hrnryn pro- 



cipitate, formed locally, redissolyes to give a deep reddish solution. 

 This contains ferric hydroxide in colloidal suspension. The 

 sodium chloride and unused ferric chloride can be separated by 

 dialysis (p. 442), and a pure colloidal suspension of the hydroxide 

 obtained. 



Ferrous hydroxide Fe(OH) 2 produced by precipitation, is 

 white when pure, but becomes quickly green and then brown by 

 oxidation. 



Oxides of Iron. When ferric hydroxide is heated, ferric 

 oxide Fe 2 3 remains as a red mass: 



2Fe(OH) 3 - Fe 2 3 + 3H 2 O f . 



It is made by calcining (roasting) ferrous carbonate, ferrous sul- 

 phide, or ferrous sulphate : 



4FeSO 4 + O 2 -> 2Fe 2 3 + 4SO 3 . 

 4FeS + 70 2 - 2Fe 2 3 + 4SO 2 . 



The oxide, when pulverized in a ball mill, gives a powder of 

 more or less brilliant red color commonly used in paints (Venetian 

 red and Indian red). That from ferrous sulphate is rouge, used 

 in polishing plate glass and lenses, and as a pigment. Yellow 

 ochre is a natural hydrated ferric oxide 2Fe 2 O 3 ,3H 2 O, which 

 acquires various depths of color during calcination, and constitutes 

 the sienna and umber used for paints. 



Magnetic Oxide of Iron Fe 3 4 is regarded as a compound of 

 ferrous and ferric oxides FeO,Fe 2 O 3 . It is formed by strongly 

 heating ferric oxide : 



6Fe 2 3 -> 4Fe 3 4 + 2 



and is formed when iron is oxidized at a high temperature by oxy- 

 gen, air (blacksmith's scale), or steam. It can be magnetized, and 

 natural specimens are often strongly magnetic (lodestone). 



