d
by hydrosulphuric acid.

10. What hydroxide studied, other than zinc hydroxide, has both acid and
basic properties?

11. Write equations showing how the following compounds of zinc may be
obtained from metallic zinc: the oxide, chloride, nitrate, carbonate,
sulphate, sulphide, hydroxide.

CHAPTER XXVI

THE ALUMINIUM FAMILY

~The family.~ The element aluminium is the most abundant member of the
group of elements known as the aluminium family; indeed, the other
members of the family--gallium, indium, and thallium--are of such rare
occurrence that they need not be separately described. The elements of
the family are ordinarily trivalent, so that the formulas for their
compounds differ from those of the elements so far studied. Their
hydroxides are practically insoluble in water and are very weak bases;
indeed, the bases are so weak that their salts are often hydrolyzed into
free base and free acid in solution. The salts formed from these bases
usually contain water of crystallization, which cannot be driven off
without decomposing them more or less.

The trivalent metals, which in addition to aluminium include also iron
and chromium, are sometimes called the _earth metals_. The name refers
to the earthy appearance of the oxides of these metals, and to the fact
that many earths, soils, and rocks are composed in part of these
substances.

ALUMINIUM

~Occurrence.~ Aluminium never occurs in the free state in nature, owing to
its great affinity for oxygen. In combined form, as oxides, silicates,
and a few other salts, it is both abundant and widely distributed, being
an essential constituent of all soils and of most rocks excepting
limestone and sandstone. Cryolite (Na_{3}AlF_{6}), found in Greenland,
and bauxite, which is an aluminium hydroxide usually mixed with some
iron hydroxide, are important minerals. It is estimated that aluminium
composes about 8% of the earth's crust. In the industries the metal is
called aluminum, but its chemical name is aluminium.

[Illustration: Fig. 82]

~Preparation.~ Aluminium was first prepared by Woehler, in 1827, by heating
anhydrous aluminium chloride with potassium:

AlCl_{3} + 3K = 3KCl + Al.

This method was tried after it was found impossible to reduce the oxide
of aluminium with carbon. The metal possessed such interesting
properties and promised to be so useful that many efforts were made to
devise a cheap way of preparing it. The method which has prov