Modern inorganic chemistry

THE ELEMENTS OF GROUP III 157

OTHER COMPOUNDS

Alums

These are double salts which have the general formula

MIMIII(SO 4 ) 2 .12H 2 O

where M^1 may be an alkali metal or ammonium, and Mm may be
aluminium, chromium, iron, manganese, cobalt and others in
oxidation state +3. kAlum' is KA1(SO 4 ) 2 .12H 2 O. They are double
salts, not complex salts, i.e. they contain the ions (for example)
K+, [A1(H 2 O) 6 ]^3 + and SO^

USES OF BORON AND ALUMINIUM

Many of the uses of boron and aluminium compounds have already
been discussed. The elements and a number of other compounds
also have important applications.

Boron

Metal borides. for example those of molybdenum and titanium, are
being increasingly used in aircraft, space craft, and high speed metal
cutting tools These borides are extremely hard and can withstand
high temperatures. The element boron is a good neutron absorber
and is used for shielding and in control rods for nuclear reactors. Its
burning characteristics lead to its use in flares.

Aluminium

Industrial apparatus and many domestic articles (for example pans
and kettles) are made from aluminium. Aluminium powder is used in
anti-corrosion paints and in explosives (for example ammonal).
Weight for weight, aluminium is a better electrical conductor than
copper, so that wires may be made from it. It is used in overhead
cables—aluminium wires being twisted round steel wires, the latter
giving greater mechanical strength. Aluminium foil is now often
used instead of tin foil for wrapping foodstuffs. Aluminium deposited
from the vapour on to glass can form excellent mirrors which do not
tarnish. Aluminium alloys are used extensively in the aircraft and
motor industries, for example as duralumin and magnalium.