282 PRINCIPLES : OF CHEMISTRY 



Chromic anhydride is a red crystalline substance, which is converted 

 into a black mass by heat ; it fuses at 190, and disengages oxygen 

 above 250, leaving a residue of chromium dioxide, CrO 2 , 6 and, on still 

 further heating, chromic oxide, Cr 2 3 . Chromic anhydride is exceed- 

 ingly soluble in water, and even attracts moisture from the air, but, as 

 was mentioned above, it does not form any definite compound with 

 water. The specific gravity of its crystals is 2 '7, and when fused it has 

 a specific gravity 2*6. The solution presents perfectly defined acid 

 properties. It liberates carbonic anhydride from carbonates ; gives 

 insoluble precipitates of the chromates with salts of barium, lead, silver, 

 and mercury. 



The action of hydrogen peroxide on a solution of chromic acid or of 

 potassium dichromate gives a blue solution, which very quickly becomes 

 colourless with the disengagement of oxygen. Barreswill showed that 

 this is due to the formation of a perchromic anhydride, Cr 2 7 , corre- 

 sponding with sulphur peroxide. This peroxide is remarkable from the 

 fact that it very easily dissolves in ether and is much more stable in 

 this solution, so that, by shaking up hydrogen peroxide mixed with a 

 small quantity of chromic acid, with ether, it is possible to transfer all 

 the blue substance formed to the ether. 6 bis 



With oxygen acids, chromic acid evolves oxygen ; for example, with 



<* This intermediate degree of oxidation, CrO 2 , may also be obtained by mixing solu- 

 tions of chromic salts with solutions of chromates. The brown precipitate formed 

 contains a compound, Cr 2 O 3) CrO3, consisting of equivalent amounts of chromic oxide 

 and anhydride. The' brown precipitate of chromium dioxide contains water. The same 

 substance is formed by the imperfect deoxidation of chromic anhydride by various redu- 

 cing agents. Chromic oxide, when heated, absorbs oxygen, and appears to give the same 

 substance. Chromic nitrate, when ignited, also gives this substance. When this sub- 

 stance is heated it first disengages water and then oxygen, chromic oxide being left. It 

 corresponds with manganese dioxide, Cr 2 0.3,CrO 3 = 3CrO 2 . Kriiger treated chromium 

 dioxide with a mixture of sodium chloride and sulphuric acid, and found that chlorine 

 gas was evolved, but that chroniyl chloride was not formed. Under the action of light, 

 a solution of chromic acid also deposits the brown dioxide. At the ordinary temperature 

 chromic anhydride leaves a brown stain upon the skin and tissues, which probably pro- 

 ceeds from a decomposition of the same kind. Chromic anhydride is soluble in alcohol 

 containing water, and this solution is decomposed "-in a similar manner by light. 

 Chromium dioxide forms K 2 CrO 4 when treated with H 2 O 2 in the presence of KHO. 



6 bis NOW that persulphuric acid H 2 S 2 O 8 is well known it might be supposed that 

 perchromic anhydride, Cr 2 O 7 , would correspond to perchromic acid, H 2 Cr 2 Oa, but as yet 

 it is not certain whether corresponding salts are formed. Pe'chard (1891) on adding an 

 excess of H 2 O 2 and baryta water to a dilute solution of CrO 2 (8 grm. per litre), observed 

 the formation of a yellow precipitate, but oxygen was disengaged at the same time and 

 the precipitate (which easily exploded when dried) was found to contain, besides an 

 admixture of BaO 2 , a compound BaCrO 5 , and this = BaO 3 + CrO 3 , and does not correspond 

 to perchromic acid. The fact of its decomposing with an explosion, and the mode of ite: 

 preparation, proves, however, that this is a similar derivative of peroxide of hydrogen to. 

 persulphuric: acid (Chapter XX.) 



