(k.) Chromium (Cr) occurs in the metallic state only in a very small quantity in meteoric iron, but is frequently found in union with oxygen, as oxide in chrome iron ore, and as chromic acid in some lead ores.
In the metallic state it is of a light grey color, with but little metallic lustre, very hard, and not very fusible. Acids do not act upon it, except the hydrofluoric; fused with nitre, it forms chromate of potassa. It is unaltered in the blowpipe flame.
Sesquioxide of Chromium (Cr2O3).—This oxide forms black crystals of great hardness, and is sometimes seen as a green powder. Its hydrate (Cr2O3 + 6HO) is of a bluish-grey color. It forms with acids two classes of isomeric salts, some of which are of a green color, and the others violet-red or amethyst. The neutral and soluble salts have an acid reaction upon blue litmus paper, and are decomposed by ignition.
Sesquioxide of chromium in the oxidation and reduction flames is unchangable. When exposed to heat, the hydrate loses its water, and gives a peculiarly beautiful flame. In the oxidation flame borax dissolves the sesquioxide of chromium slowly to a yellow bead (chromic acid) which is yellowish green when cold. Upon the addition of more of the oxide, the bead is dark red while hot, but changes to green as it becomes cold.
In the reduction flame the bead is of a beautiful green color, both while hot and when cold. It is here distinguished from vanadic acid, which gives a brownish or yellow bead while hot.
With microcosmic salt it fuses in the oxidation flame to a clear yellow bead, which appears, as it cools, of a dirty-green, color, but upon being cool is of a fine green color. If there be a superabundance of the oxide, so that the microcosmic salt cannot dissolve it, the bead swells up, and is converted into a foamy mass, in consequence of the development of gases.
In the reduction flame it fuses to a fine green bead. The addition of a little tin renders the green still deeper.
Sesquioxide of chromium fuses with carbonate of soda upon platinum foil to a brown or yellow bead, which, upon cooling, appears of a lighter color and transparent (chromate of sodium).
When fused with soda upon charcoal, the soda is absorbed, and the green oxide is left upon it, but is never reduced to the metallic state.
Chromic Acid (CrO3) crystallizes in the form of deep ruby red needles. It is decomposed into sesquioxide and oxygen when heated. This decomposition is attended with a very lively emission of light, but this is not the case if the chromic acid has been attained by the coöperation of an aqueous solution, unless the reduction is effected in the vapor of ammonia. Before the blowpipe chromic acid produces the same reactions as the sesquioxide.