130 GROUPS! AND IIacids to give salts; beryllium oxide is inert and almost insoluble in
water or in acids.
Group 1 elements, except lithium, form peroxides M2O 2 with
excess oxygen, and potassium, rubidium and caesium will form
super oxides MO 2. These per- and super- oxides are best prepared
by passing oxygen into a solution of the metal in liquid ammonia.
It is believed that the large ions Q\~ and O^ are only stable in
lattices with larger cations—hence lithium (small cation) forms only
the normal oxide Li 2 O. The elements of Group II also form per-
oxides.
The hydroxides M*OH are all soluble in water, in which they
behave as strong bases, for example
KOH-+K+ + OH"The hydroxides M"(OH) 2 are generally less soluble and are of lower
base strength. The Group I hydroxides are almost unique in
possessing good solubility—most metal hydroxides are insoluble or
sparingly soluble; hence sodium hydroxide and, to a lesser extent
potassium hydroxide, are widely used as sources of the hydroxide
ion OH~ both in the laboratory and on a large scale.
Sodium hydroxide is manufactured by electrolysis of concentrated
aqueous sodium chloride; the other product of the electrolysis,
chlorine, is equally important and hence separation of anode and
cathode products is necessary. This is achieved either by a diaphragm
(for example in the Hooker electrolytic cell) or by using a mercury
cathode which takes up the sodium formed at the cathode as an
amalgam (the Kellner-Solvay cell). The amalgam, after removal from
the electrolyte cell, is treated with water to give sodium hydroxide
and mercury. The mercury cell is more costly to operate but gives a
purer product.
Potassium hydroxide is similar to sodium hydroxide but is a
stronger base; it is also more soluble in alcohol and the solution is
sometimes used as a reagent ('alcoholic potash^5 ). The other hydrox-
ides of Group I are similar, increasing in base strength down the
group*; all are hygroscopic solids which attack the skin- hence the
old names, "caustic soda' (NaOH), "caustic potash' (KOH)—and
react with carbon dioxide in the air to give carbonates:
2OH~ + CO 2 -* CC>r + H 2 O
With excess carbon dioxide, i.e. if the gas is passed through a
solution of the hydroxide, a hydrogencarbonate is formed:
- With the smaller cations (Li". Na*) there is some association of the OH" ion
with the cation in solution, and this results in a lower base strength.