}S + 3O = H_{2}O + SO_{2}.

When there is not enough oxygen for both the sulphur and the hydrogen,
the latter element combines with the oxygen and the sulphur is set free:

H_{2}S + O = H_{2}O + S.

3. _Reducing action._ Owing to the ease with which hydrosulphuric acid
decomposes and the strong affinity of both sulphur and hydrogen for
oxygen, the substance is a strong reducing agent, taking oxygen away
from many substances which contain it.

4. _Action on metals._ Hydrosulphuric acid acts towards metals in a way
very similar to water. Thus, when it is passed over heated iron in a
tube, the reaction is represented by the equation

3Fe + 4H_{2}S = Fe_{3}S_{4} + 8H.

Water in the form of steam, under similar circumstances, acts according
to the equation

3Fe + 4H_{2}O = Fe_{3}O_{4} + 8H.

~Salts of hydrosulphuric acid,--sulphides.~ The salts of hydrosulphuric
acid, called sulphides, form an important class of salts. Many of them
are found abundantly in nature, and some of them are important ores.
They will be frequently mentioned in connection with the metals.

Most of the sulphides are insoluble in water, and some of them are
insoluble in acids. Consequently, when hydrosulphuric acid is passed
into a solution of a salt, it often happens that a sulphide is
precipitated. With copper chloride the equation is

CuCl_{2} + H_{2}S = CuS + 2HCl.

Because of the fact that some metals are precipitated in this way as
sulphides while others are not, hydrosulphuric acid is extensively used
in the separation of the metals in the laboratory.

~Explanation of the reaction.~ When hydrosulphuric acid and
copper chloride are brought together in solution, both copper
and sulphur ions are present, and these will come to an
equilibrium, as represented in the equation

Cu^{+} + S^{-} <--> CuS.

Since copper sulphide is almost insoluble in water, as soon as
a very small quantity has formed the solution becomes
supersaturated, and the excess keeps precipitating until nearly
all the copper or sulphur ions have been removed from the
solution. With some other ions, such as iron, the sulphide
formed does not saturate the solution, and no precipitate
results.

OXIDES OF SULPHUR

Sulphur forms two well-known compounds with oxygen: sulphur dioxide
(SO_{2}), sometimes called sulphurous anhydride; and sulphur trioxide
(SO_{3}), frequently called sulphur