hlorine is set free.
Thus, when a solution of hydrochloric acid acts upon manganese dioxide
part of the chlorine is set free:

MnO_{2} + 4HCl = MnCl_{2} + 2H_{2}O + 2Cl.

~Aqua regia.~ It has been seen that when nitric acid acts as an oxidizing
agent it usually decomposes, as represented in the equation

2HNO_{3} = H_{2}O + 2NO + 3O.

The oxygen so set free may act on hydrochloric acid:

6HCl + 3O = 3H_{2}O + 6Cl.

The complete equation therefore is

2HNO_{3} + 6HCl = 4H_{2}O + 2NO + 6Cl.

When concentrated nitric and hydrochloric acids are mixed this reaction
goes on slowly, chlorine and some other substances not represented in
the equation being formed. The mixture is known as _aqua regia_ and is
commonly prepared by adding one volume of nitric acid to three volumes
of hydrochloric acid. It acts more powerfully upon metals and other
substances than either of the acids separately, and owes its strength
not to acid properties but to the action of the nascent chlorine which
it liberates. Consequently, when it acts upon metals such as gold it
converts them into chlorides, and the reaction can be represented by
such equations as

Au + 3Cl = AuCl_{3}.

~Salts of hydrochloric acid,--chlorides.~ The chlorides of all the metals
are known and many of them are very important compounds. Some of them
are found in nature, and all can be prepared by the general method of
preparing salts. Silver chloride, lead chloride, and mercurous chloride
are insoluble in water and acids, and can be prepared by adding
hydrochloric acid to solutions of compounds of the respective elements.
While the chlorides have formulas similar to the fluorides, their
properties are often quite different. This is seen in the solubility of
the salts. Those metals whose chlorides are insoluble form soluble
fluorides, while many of the metals which form soluble chlorides form
insoluble fluorides.

~Compounds of chlorine with oxygen and hydrogen.~ Chlorine combines with
oxygen and hydrogen to form four different acids. They are all quite
unstable, and most of them cannot be prepared in pure form; their salts
can easily be made, however, and some of them will be met with in the
study of the metals. The formulas and names of these acids are as
follows:

HClO hypochlorous acid.

HClO_{2} chlorous acid.

HClO_{3} chloric acid.

HClO_{4} perchloric acid.

~Oxides of chlorine.~ Two oxides are known, ha