alums, though they contain no aluminium. They all crystallize in
octahedra and contain twelve molecules of water of crystallization. The
alums most frequently prepared are the following:

Potassium alum KAl(SO_{4})_{2}.12H_{2}O.
Ammonium alum NH_{4}Al(SO_{4})_{2}.12H_{2}O.
Ammonium iron alum NH_{4}Fe(SO_{4})_{2}.12H_{2}O.
Potassium chrome alum KCr(SO_{4})_{2}.12H_{2}O.

An alum may therefore be regarded as a compound derived from two
molecules of sulphuric acid, in which one hydrogen atom has been
displaced by the univalent alkali atom, and the other three hydrogen
atoms by an atom of one of the trivalent metals, such as aluminium,
iron, or chromium.

Very large, well-formed crystals of an alum can be prepared by
suspending a small crystal by a thread in a saturated solution
of the alum, as shown in Fig. 84. The small crystal slowly
grows and assumes a very perfect form.

[Illustration: Fig. 84]

~Other salts of aluminium.~ While aluminium hydroxide forms fairly stable
salts with strong acids, it is such a weak base that its salts with weak
acids are readily hydrolyzed. Thus, when an aluminium salt and a soluble
carbonate are brought together in solution we should expect to have
aluminium carbonate precipitated according to the equation

3Na_{2}CO_{3} + 2AlCl_{3} = Al_{2}(CO_{3})_{3} + 6NaCl.

But if it is formed at all, it instantly begins to hydrolyze, the
products of the hydrolysis being aluminium hydroxide and carbonic acid,

Al_{2}(CO_{3})_{3} + 6H_{2}O = 2Al(OH)_{3} + 3H_{2}CO_{3}.

Similarly a soluble sulphide, instead of precipitating aluminium
sulphide (Al_{2}S_{3}), precipitates aluminium hydroxide; for hydrogen
sulphide is such a weak acid that the aluminium sulphide at first formed
hydrolyzes at once, forming aluminium hydroxide and hydrogen sulphide:

3Na_{2}S + 2AlCl_{3} + 6H_{2}O = 2Al(OH)_{3} + 6NaCl + 3H_{2}S.

~Alum baking powders.~ It is because of the hydrolysis of aluminium
carbonate that alum is used as a constituent of some baking powders. The
alum baking powders consist of a mixture of alum and sodium hydrogen
carbonate. When water is added the two compounds react together, forming
aluminium carbonate, which hydrolyzes into aluminium hydroxide and
carbonic acid. The carbon dioxide from the latter escapes through the
dough and in so doing raises it into a porous condition, which is the
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