{4} + 3H_{2}O + 5O.

Under ordinary conditions, however, neither one of these reactions takes
place except in the presence of a third substance which is capable of
oxidation. The oxygen is not given off in the free state, as the
equations show, but is used up in effecting oxidation.

Potassium permanganate is particularly valuable as an oxidizing agent
not only because it acts readily either in acid or in alkaline solution,
but also because the reaction takes place so easily that often it is not
even necessary to heat the solution to secure action. The substance
finds many uses in the laboratory, especially in analytical work. It is
also used as an antiseptic as well as a disinfectant.

CHROMIUM

~Occurrence.~ The ore from which all chromium compounds are made is
chromite, or chrome iron ore (FeCr_{2}O_{4}). This is found most
abundantly in New Caledonia and Turkey. The element also occurs in small
quantities in many other minerals, especially in crocoisite (PbCrO_{4}),
in which mineral it was first discovered.

~Preparation.~ Chromium, like manganese, is very hard to reduce from its
ores, owing to its great affinity for oxygen. It can, however, be made
by the same methods which have proved successful with manganese.
Considerable quantities of an alloy of chromium with iron, called
ferrochromium, are now produced for the steel industry.

~Properties.~ Chromium is a very hard metal of about the same density as
iron. It is one of the most infusible of the metals, requiring a
temperature little short of 3000 deg. for fusion. At ordinary temperatures
air has little action on it; at higher temperatures, however, it burns
brilliantly. Nitric acid has no action on it, but hydrochloric and
dilute sulphuric acids dissolve it, liberating hydrogen.

~Compounds containing chromium as a base-forming element.~ While chromium
forms two series of salts, chromous salts are difficult to prepare and
are of little importance. The most important of the chromic series are
the following:

Chromic hydroxide Cr(OH)_{3}.
Chromic chloride CrCl_{3}.6H_{2}O.
Chromic sulphate Cr_{2}(SO_{4})_{3}.
Chrome alums

~Chromic hydroxide~ (Cr(OH)_{3}). This substance, being insoluble, can be
obtained by precipitating a solution of the chloride or sulphate with a
soluble hydroxide. It is a greenish substance which, like aluminium
hydroxide, dissolves in alkalis, forming soluble salts.

~Dehydration of chr