be heated with carbon in an electric
furnace to give ferrochrome, which
is used in making alloy steels. The
metal is also used as a shiny decora-
tive electroplated coating and in the
manufacture of certain chromium
compounds.
At normal temperatures the metal
is corrosion-resistant. It reacts with
dilute hydrochloric and sulphuric
acids to give chromium(II) salts.
These readily oxidize to the more sta-
ble chromium(III) salts. Chromium
also forms compounds with the +6
oxidation state, as in chromates,
which contain the CrO 4 2–ion. The el-
ement was discovered in 1797 by
Vauquelin.
A
- Information from the WebElements site
chromium dioxide See chro-
mium(iv) oxide.
chromium(II) oxide A black insol-
uble powder, CrO. Chromium(II)
oxide is prepared by oxidizing
chromium amalgam with air. At high
temperatures hydrogen reduces it to
the metal.
chromium(III) oxide (chromium
sesquioxide)A green crystalline
water-insoluble salt, Cr 2 O 3 ; r.d. 5.21;
m.p. 2435°C; b.p. 4000°C. It is ob-
tained by heating chromium in a
stream of oxygen or by heating am-
monium dichromate. The industrial
preparation is by reduction of
sodium dichromate with carbon.
Chromium(III) oxide is amphoteric,
dissolving in acids to give chro-
mium(III) ions and in concentrated
solutions of alkalis to give chromites.
It is used as a green pigment in glass,
porcelain, and oil paint.
chromium(IV) oxide (chromium
dioxide)A black insoluble powder,
CrO 2 ; m.p. 300°C. It is prepared by
the action of oxygen on
chromium(VI) oxide or chromium(III)
oxide at 420–450°C and 200–300 at-
mospheres. The compound is unsta-
ble.chromium(VI) oxide (chromium
trioxide; chromic anhydride) A red
compound, CrO 3 ; rhombic; r.d. 2.70;
m.p. 196°C. It can be made by careful
addition of concentrated sulphuric
acid to an ice-cooled concentrated
aqueous solution of sodium dichro-
mate with stirring. The mixture is
thenÜltered through sintered glass,
washed with nitric acid, then dried at
120 °C in a desiccator.
Chromium(VI) oxide is an ex-
tremely powerful oxidizing agent,
especially to organic matter; it im-
mediately inÛames ethanol. It is an
acidic oxide and dissolves in water to
form ‘chromic acid’, a powerful oxi-
dizing agent and cleansingÛuid for
glassware. At 400°C, chromium(VI)
oxide loses oxygen to give
chromium(III) oxide.chromium potassium sulphate
A red crystalline solid,
K 2 SO 4 .Cr 2 (SO 4 ) 3 .24H 2 O; r.d. 1.91. It is
used as a mordant See also alums.chromium sesquioxide See
chromium(iii) oxide.chromium steel Any of a group of
*stainless steels containing 8–25% of
chromium. A typical chromium steel
might contain 18% of chromium, 8%
of nickel, and 0.15% of carbon.
Chromium steels are highly resistant
to corrosion and are used for cutlery,
chemical plant, ball bearings, etc.chromium trioxide See
chromium(vi) oxide.chromophore A group causing col-
oration in a *dye. Chromophores are
generally groups of atoms having de-
localized electrons.chromous compounds Com-
pounds containing chromium in its125 chromous compounds
c