158 THE ELEMENTS OF GROUP III
Gallium, indium, thallium
Inlermetallic compounds with gallium are used as semiconductors.
Indium is used to coat other metals to protect against corrosion,
especially in engine bearings; it is also a constituent of low-metal
alloys used in safety sprinklers. The toxicity of thallium compounds
has limited the use of the metal, but it does find use as a constituent
of high-endurance alloys for bearings.
TESTS FOR BORON AND ALUMINIUM
Boron
Volatile boron compounds burn with a green flame. If a solid borate
is mixed with methanol and concentrated sulphuric acid the
volatile compound boron trimethoxide, B(OCH 3 ) 3 , is formed and
ignition of the alcohol therefore produces a green flame:
3CH 3 OH + H 3 BO 3 -> B(OCH 3 ) 3 + 3H 2 O
(The water formed is taken up by the concentrated sulphuric acid.)
Aluminium
(1) Addition of ammonium hydroxide to a solution of an alu-
minium salt gives a white gelatinous precipitate of aluminium
hydroxide, A1(OH) 3 , insoluble in excess. Sodium hydroxide gives
the same precipitate, but in this case, it does dissolve in excess.
(2) Addition of ammonium hydroxide to an aluminium salt in
solution in presence of alizarin, gives a pink precipitate.
QUESTIONS
- The properties of the head element of a main group in the
periodic table resemble those of the second element in the next
group. Discuss this 'diagonal relationship' with particular reference
to (a) lithium and magnesium, (b) beryllium and aluminium.
(Lverpool B.Sc, Part I)
- Outline the extraction of pure aluminium from bauxite. (Details
of the purification of the bauxite are not required.)
