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 decorative 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 stable chromium(III) salts. Chromium
also forms compounds with the +6
oxidation state, as in chromates,
which contain the CrO 4
2– ion. The element was discovered in 1797 by
Vauquelin.
A
• Information from the WebElements site
chromium dioxide See chromium(iv) oxide.
chromium(II) oxide A black insoluble 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 obtained by heating chromium in a
stream of oxygen or by heating ammonium dichromate. The industrial
preparation is by reduction of
sodium dichromate with carbon.
Chromium(III) oxide is amphoteric,
dissolving in acids to give chromium(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 atmospheres. The compound is unstable.
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 dichromate with stirring. The mixture is
then Ültered through sintered glass,
washed with nitric acid, then dried at
120°C in a desiccator.
Chromium(VI) oxide is an extremely powerful oxidizing agent,
especially to organic matter; it immediately inÛames ethanol. It is an
acidic oxide and dissolves in water to
form ‘chromic acid’, a powerful oxidizing agent and cleansing Ûuid 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 coloration in a *dye. Chromophores are
generally groups of atoms having delocalized electrons.
chromous compounds Compounds containing chromium in its
125
chromous compounds
c
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furnace to give ferrochrome, which
is used in making alloy steels. The
metal is also used as a shiny decorative 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 stable chromium(III) salts. Chromium
also forms compounds with the +6
oxidation state, as in chromates,
which contain the CrO 4
2– ion. The element was discovered in 1797 by
Vauquelin.
A
• Information from the WebElements site
chromium dioxide See chromium(iv) oxide.
chromium(II) oxide A black insoluble 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 obtained by heating chromium in a
stream of oxygen or by heating ammonium dichromate. The industrial
preparation is by reduction of
sodium dichromate with carbon.
Chromium(III) oxide is amphoteric,
dissolving in acids to give chromium(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 atmospheres. The compound is unstable.
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 dichromate with stirring. The mixture is
then Ültered through sintered glass,
washed with nitric acid, then dried at
120°C in a desiccator.
Chromium(VI) oxide is an extremely powerful oxidizing agent,
especially to organic matter; it immediately inÛames ethanol. It is an
acidic oxide and dissolves in water to
form ‘chromic acid’, a powerful oxidizing agent and cleansing Ûuid 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 coloration in a *dye. Chromophores are
generally groups of atoms having delocalized electrons.
chromous compounds Compounds containing chromium in its
125
chromous compounds
c
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