th the oxygen in the steam, setting free the
hydrogen, which is collected over water. The gas which first
passes over is mixed with the air previously contained in the
flask and tube, and is allowed to escape, _since a mixture of
hydrogen with oxygen or air explodes violently when brought in
contact with a flame_. It is evident that the flask A must be
disconnected from the tube before the heat is withdrawn.

That the gas obtained is different from air and oxygen may be
shown by holding a bottle of it mouth downward and bringing a
lighted splint into it. The hydrogen is ignited and burns with
an almost colorless flame.

[Illustration Fig. 9]

~Preparation from acids~ (_usual laboratory method_). While hydrogen can
be prepared from water, either by the action of the electric current or
by the action of certain metals, these methods are not economical and
are therefore but little used. In the laboratory hydrogen is generally
prepared from compounds known as acids, all of which contain hydrogen.
When acids are brought in contact with certain metals, the metals
dissolve and set free the hydrogen of the acid. Although this reaction
is a quite general one, it has been found most convenient in preparing
hydrogen by this method to use either zinc or iron as the metal and
either hydrochloric or sulphuric acid as the acid. Hydrochloric acid is
a compound consisting of 2.77% hydrogen and 97.23% chlorine, while
sulphuric acid consists of 2.05% hydrogen, 32.70% sulphur, and 65.25%
oxygen.

The changes which take place in the preparation of hydrogen from zinc
and sulphuric acid (diluted with water) may be represented as follows:

_ _ _ _
| hydrogen |(sulphuric | zinc |(zinc
zinc + | sulphur | acid) = | sulphur | sulphate) + hydrogen
|_oxygen _| |_oxygen _|

In other words, the zinc has taken the place of the hydrogen in
sulphuric acid. The resulting compound contains zinc, sulphur, and
oxygen, and is known as zinc sulphate. This remains dissolved in the
water present in the acid. It may be obtained in the form of a white
solid by evaporating the liquid left after the metal has passed into
solution.

When zinc and hydrochloric acid are used the following changes take
place:

_ _ _ _
| hydrogen |(hydrochloric | zinc |(zinc
zinc + |_chlorine_| acid)