ich after drying over sulphuric
acid, has the composition Cr2O3.7H2O, and this after being heated to
200 deg. C. in a current of hydrogen leaves a residue of composition
CrO.OH or Cr2O3.H2O which occurs naturally as chrome ochre. Other
hydrated oxides such as Cr2O3.2H2O have also been described. Chromium
trioxide, CrO3, is obtained by adding concentrated sulphuric acid to a
cold saturated solution of potassium bichromate, when it separates in
long red needles; the mother liquor is drained off and the crystals
are washed with concentrated nitric acid, the excess of which is
removed by means of a current of dry air. It is readily soluble in
water, melts at 193 deg. C., and is decomposed at a higher temperature
into chromium sesquioxide and oxygen; it is a very powerful oxidizing
agent, acting violently on alcohol, converting it into acetaldehyde,
and in glacial acetic acid solution converting naphthalene and
anthracene into the corresponding quinones. Heated with concentrated
hydrochloric acid it liberates chlorine, and with sulphuric acid it
liberates oxygen. Gaseous ammonia passed over the oxide reduces it to
the sesquioxide with formation of nitrogen and water. Dissolved in
hydrochloric acid at -20 deg., it yields with solutions of the alkaline
chlorides compounds of the type MCl.CrOCl3, pointing to pentavalent
chromium. For salts of this acid-forming oxide and for perchromic acid
see BICHROMATES.

The chromites may be looked upon as salts of chromium sesquioxide with
other basic oxides, the most important being chromite (q.v.).

Chromous chloride, CrCl2, is prepared by reducing chromic chloride in
hydrogen; it forms white silky needles, which dissolve in water giving
a deep blue solution, which rapidly absorbs oxygen, forming basic
chromic salts, and acts as a very strong reducing agent. The bromide
and iodide are formed in a similar manner by heating the metal in
gaseous hydrobromic or hydriodic acids.

Chromous sulphate, CrSO4.7H2O, isomorphous with ferrous sulphate,
results on dissolving the metal in dilute sulphuric acid or, better,
by dissolving chromous acetate in dilute sulphuric acid, when it
separates in blue crystals on cooling the solution. On pouring a
solution of chromous chloride into a saturated solution of sodium
acetate, a red crystalline precipitate of chromous acetate is
produced; this is much more permanent in air than the ot