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.