= |_chlorine_| chloride) + hydrogen

When iron is used the changes which take place are exactly similar to
those just given for zinc.

[Illustration Fig. 10.]

~Directions for preparing hydrogen from acids.~ The preparation
of hydrogen from acids is carried out in the laboratory as
follows: The metal is placed in a flask or wide-mouthed bottle
A (Fig. 10) and the acid is added slowly through the funnel
tube B. The metal dissolves in the acid, while the hydrogen
which is liberated escapes through the exit tube C and is
collected over water. It is evident that the hydrogen which
passes over first is mixed with the air from the bottle A.
Hence care must be taken not to bring a flame near the exit
tube, since, as has been stated previously, such a mixture
explodes with great violence when brought in contact with a
flame.

~Precautions.~ Both sulphuric acid and zinc, if impure, are
likely to contain small amounts of arsenic. Such materials
should not be used in preparing hydrogen, since the arsenic
present combines with a portion of the hydrogen to form a very
poisonous gas known as arsine. On the other hand, chemically
pure sulphuric acid, i.e. sulphuric acid that is entirely free
from impurities, will not act upon chemically pure zinc. The
reaction may be started, however, by the addition of a few
drops of a solution of copper sulphate or platinum
tetrachloride.

~Physical properties.~ Hydrogen is similar to oxygen in that it is a
colorless, tasteless, odorless gas. It is characterized by its extreme
lightness, being the lightest of all known substances. One liter of the
gas weighs only 0.08984 g. On comparing this weight with that of an
equal volume of oxygen, viz., 1.4285 g., the latter is found to be 15.88
times as heavy as hydrogen. Similarly, air is found to be 14.38 times as
heavy as hydrogen. Soap bubbles blown with hydrogen rapidly rise in the
air. On account of its lightness it is possible to pour it upward from
one bottle into another. Thus, if the bottle A (Fig. 11) is filled
with hydrogen, placed mouth downward by the side of bottle _B_, filled
with air, and is then gradually inverted under B as indicated in the
figure, the hydrogen will flow upward into bottle _B_, displacing the
air. Its presence in bottle B may then be shown by bringing a lighted
splint to the mouth of the bottle, when the