286 PRINCIPLES OF CHEMISTRY 



pounds of the oxide itself, CrX 3 or Cr 2 X 6 , like alumina, which it 

 resembles in forming a feeble base easily giving double and basic salts, 

 which are either green or violet. 



to dry it perfectly before it reaches the tube. On heating the portion of the tube in 

 which the mixture is placed and passing chlorine through, a slightly volatile sublimate of 

 chromic chloride, CrCl 3 or Cr 2 Cl 6 , is formed. This substance forms violet tabular 

 crystals, which may be distilled in dry chlorine without change, but which, however, re- 

 quire a red heat for their volatilisation. "These crystals are greasy to the touch and in- 

 soluble in water, but if they be powdered and boiled in water for a long time they pass 

 into a green solution. Strong sulphuric acid does not act on the anhydrous salt, or 

 only acts with exceeding slowness, like water. Even aqua regia and other acids do not 

 act on the crystals, and alkalis only show a very feeble action. The specific gravity of 

 the crystals is 2'99. When fused with sodium carbonate and nitre they give sodium 

 chloride and potassium chromate, and when ignited in air they form green chromic oxide 

 and evolve chlorine. On jgnition in a stream of ammonia, chromic chloride forms 

 sal-ammoniac and chromium nitride, CrN (analogous to the nitridesBN,A!N). Mosberg and 

 Peligot showed that when chromic chloride is ignited in hydrogen, it parts with one-third 

 of its chlorine, forming chromous chloride, CrCl 2 that is, there is formed from a com- 

 pound corresponding with chromic oxide, Cr 2 O 3 , a compound answering to the suboxide, 

 chromous oxide, CrO just as hydrogen converts ferric chloride into ferrous chloride with 

 the aid of heat. Chromous chloride, CrCl 2 , forms colourless crystals easily soluble in 

 water, which in dissolving evolve a considerable amount of heat, and form a blue liquid, 

 capable of absorbing oxygen from the air with great facility, being converted thereby 

 into a chromic compound. 



The blue solution of chromous chloride may also be obtained by the action of metallic 

 zinc on the green solution of the hydrated chromic chloride ; the zinc in this case takes 

 up chlorine just as the hydrogen did. It must be employed in a large excess. Chromic 

 oxide is also formed in the action of zinc on chromic chloride, and if the solution remain 

 for a long time in contact with the zinc the whole of the chromium, is converted into 

 chromic oxychloride. Other chromic salts are also reduced by zinc into chromous salts, 

 CrX 2 , just as the ferric salts FeX 3 are concerted into ferrous salts FeX 2 by it. The 

 chromous salts are exceedingly unstable and easily oxidise and pass into chromic salts; 

 hence the reducing power of these salts is very great. From cupric salts they -separate 

 cuprous salts, from stannous salts they precipitate metallic tin, they reduce mercuric 

 salts into mercurous and ferric into ferrous salts. Moreover, they absorb oxygen from 

 the air directly. With potassium chromate they give a brown precipitate of chromium 

 dioxide or of chromic oxide, according to the relative amounts of the substances taken : 

 Cr0 3 + CrO = 2CrO 2 or CrO 3 + 3CrO = 2Cr 2 Or. Aqueous ammonia gives a blue precipi- 

 tate, and in the presence of ammoniacal salts a blue liquid is obtained which turns red 

 in the air from oxidation. This is accompanied by the formation of compounds analo- 

 gous to those given by cobalt (Chapter XXII.) A solution of chromous chloride with a 

 hot saturated solution of sodium acetate, C 2 H 3 NaO 2 , gives, on coojing, transparent red 

 crystals of chromous acetate, C 4 H<jCrQ 4 ,H 2 O. This salt is also a powerful reducing 

 agent, but may be kept for a long time in a vessel full of carbonic anhydride, 



The insoluble anhydrous chromic chloride CrCl 3 very easity passes into solution in 

 the presence of a trace (0'004) of chromous chloride CrCl 2 . This remarkable phe- 

 nomenon was observed by Peligot and explained by Lb'wel in the following manners 

 chromous chloride, as a lower stage of oxidation, is capable of absorbing both oxygen 

 and chlorine, combining with various substances. It is able to decompose many 

 chlorides by taking up chlorine from them; thus it precipitates mercurous chloride from 

 a solution of mercuric chloride, and in so doing passes into chromic chloride : 2CrCl 2 

 + 2HgCl 2 = Cr 2 Cl 6 + 2HgCl. Let us suppose that the same phenomenon takes p\&ce 

 when the anhydrous chromic chloride is mixed with a solution of chromous chloride,, 

 The latter will then take up a portion, of the chlorine of the former, and pass into % 



