150 THE ELEMENTS OF GROUP IIIAluminium oxide, alumina A1 2 O 3Aluminium oxide occurs naturally as emery (an impure form) and
as corundum. Corundum is a crystalline form which may be coloured
by traces of impurity, for example as ruby (red) and sapphire (blue).
Small synthetic rubies and sapphires have been made by heating
alumina with the colouring oxide in an oxy-hydrogen flame.
Aluminium oxide may be prepared in the laboratory by heating
the hydroxide (p. 151) or by heating powdered aluminium in air,
when the oxide is formed together with some nitride. The reaction:
4A1 4- 3O 2 -> 2A1 2 O 3
is strongly exothermic and aluminium can be used to reduce some
other metallic oxides to the metal, for example manganese, chromium
and iron:
Fe 2 O 3 + 2A1 -» 2Fe + A1 2 O 3
Reduction by aluminium has been used to produce molten iron in
situ for welding steel and as a method of extracting metals.
Aluminium oxide is a white solid, insoluble in water, with a very
high melting point. If heated above red heat, it becomes insoluble
in acids and alkalis, and can only be brought into solution by first
fusing it with sodium or potassium hydroxide when an aluminate
is formed.
Alumina exists in several different crystalline forms. These have
different capacities for absorbing other substances on to the surface,
from solution. If a mixture of coloured organic substances in solution
is passed through a vertical glass tube packed with powdered
alumina, the various substances separate out as coloured zones
along the tube, and are thereby separated. Chlorophyll can be
separated into its four constituents by this method. This was an
early example of chromatography. Alumina refractories containing
more than 45% A1 2 O 3 have high resistance to abrasion and attack
by acids and are being used where ability to withstand high tem-
peratures is essential. They have a working range up to 2000 K.
Aluminium hydroxide
A white gelatinous precipitate of aluminium hydroxide is obtained
when an alkali is added to an aqueous solution of an aluminium
salt. Addition of an excess of caustic alkali causes the precipitate to
redissolve, the whole process being reversed by the addition of a
strong acid: the actual substance present at any time depending on