530 SMITH'S INTERMEDIATE CHEMISTRY 



The metal is easily obtained, by reducing chromic oxide O 2 3 

 with aluminium (Goldschmidt process, p. 468). 



Properties. Chromium is a lustrous crystalline metal. It 

 does not tarnish in air. It displaces hydrogen, however, from 

 dilute acids, giving chromous salts CrCl 2 , CrS0 4 , etc. Chromium 

 (3 per cent) gives a hard steel, and with nickel is used in armor- 

 piercing shells and in armor plate. 



Alloys which, although composed entirely of active metals, 

 are hardly affected even by boiling acids (including nitric acid), 

 usually contain chromium (e.g. 60 per cent Cr, 36 per cent Fe, 

 4 per cent Mo, no C). "Stainless" cutlery contains 12 to 14 

 per cent of chromium. 



The Chemical Relations of the Element. Chromium 

 gives four classes of compounds, and most of them are colored 

 substances (Greek, chroma, color). The chromates are derived 

 from chromic acid H 2 Cr0 4 , which is analogous to sulphuric acid 

 H 2 SO 4 . The free acid, however, is itself unstable, and leaves the 

 anhydride Cr0 3 when its solution is evaporated. The oxide and 

 hydroxide in which the element is trivalent, namely Cr 2 Oa and 

 Cr(OH) 3 , are weakly basic and still more weakly acidic. Hence 

 we have chromic salts such as CrCl 3 and Cr 2 (S0 4 )3 which are 

 somewhat hydrolyzed, but no carbonate, and no sulphide which is 

 stable in water. The compounds in which the same hydroxide 

 acts as an acid are the chromites, and are derived from the less 

 completely hydrated form of the oxide CrO(OH). Potassium 

 chromite K.CrO 2 is more easily hydrolyzed, however, than is 

 potassium zincate or potassium aluminate. Finally, the chro- 

 mous salts, such as CrCl 2 and CrS0 4 , correspond to chromous 

 hydroxide Cr(OH) 2 , in which the element is bivalent. This 

 hydroxide is more distinctly basic than is chromic hydroxide, and 

 forms a carbonate and sulphide which can be precipitated in 

 aqueous solution. The chromous salts resemble the ferrous salts 

 in being easily oxidized by the air. 



