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Chromous Oxide, CrO

Chromous Oxide, CrO, is obtained by the oxidation in air of chromium amalgam; or, preferably, by the action of dilute nitric acid upon chromium amalgam. It is a black powder which inflames when struck with a pestle, or when heated in the air, but not in a vacuum, and burns, forming the sesquioxide. It is insoluble in nitric acid and in dilute sulphuric acid, but in hydrochloric acid it dissolves, forming a blue solution and liberating hydrogen in accordance with the equation:

3CrO + 8HCl = CrCl2 + 2CrCl3 + 3H2O + H2.

When heated to 1000° C. in a stream of carbon monoxide it is converted into a mixture of oxide and carbide, while hydrogen under the same conditions reduces it to the metal.

When an air-free solution of caustic potash is added to a solution of chromous chloride in absence of air, a brownish-yellow precipitate of chromous hydroxide, Cr(OH)2, is obtained which may be dried over sulphuric acid. On heating in the presence of an inert gas this does not yield chromous oxide, since it decomposes according to the equation:

2Cr(OH)2 = Cr2O3 + H2 + H2O.

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