CHAPTER VIII. HYDROGEN.

31. Preparation.

Experiment 20.--Prepare apparatus as for making O. Be sure that
the cork perfectly fits both d.t. and t.t., or the H will escape.
Cover 5 g. granulated Zn, in the t.t., with 10 cc. H2O, and add 5
cc. chlorhydric acid, HCl. Adjust as for O (Fig. 7), except that
no heat is to be applied. If the action is not brisk enough, add
more HCl. Collect several receivers of the gas over water, adding
small quantities of HCl when necessary. Observe the black
floating residuum; it is carbon, lead, etc. With a glass plate
remove the receivers, keeping them inverted (Fig. 11), or the H
will escape.

32. The Chemical Change is as follows:--

Zinc + hydrogen chloride = zinc chloride + hydrogen.

Zn + 2 HCl = ZnCl2 + 2H.

Complete by adding the weights, and explain. Notice that the
water does not take part in the change; it is added to dissolve
the ZnCl2 formed, and thus keep it from coating the Zn and
preventing further action of the acid. Note also that Zn has
simply changed places with H, one atom of the former having
driven off two atoms of the latter. The H, having nothing to
unite with, is set free as a gas, and collected over water. Of
course Zn must have a stronger chemical affinity for Cl than H
has, or the change could not have taken place. Why one Zn atom
replaces two H atoms will be explained later, asfar as an
explanation is possible. This equation, should be studied
carefully, as a type of all equations. The left-hand member shows
what were taken, i.e. the factors; the right-hand shows what were
obtained, i.e. the products. H2SO4 might have been used instead
of HCl. In that case the reaction, or equation, would have been:
--

Zinc + hydrogen sulphate = zinc sulphate + hydrogen.

Zn + H2SO4 = ZnSO4 + 2H.

Iron might have been used instead of zinc, in which case the
reactions would have been:--

Iron + hydrogen chloride = iron chloride + hydrogen.

Fe + 2 HCl = FeCl2 + 2 H.

Iron + hydrogen sulphate = iron sulphate + hydrogen.

Fe + H2SO4 = FeSO4 + 2 H.

Write the weights and explain the equations. The latter should be
memorized.

33. Properties.

Experiment 21.--Lift with the left hand a receiver of H, still
inverted, and insert a burning splinter with the right (Fig. 12).
Does the splinter continue to burn? Does the gas burn? If so,
where? Is the light brilliant? Note the color of the flame. Is
there any explosion? Try this experiment with several receivers.
Is the gas a supporter of combustion? i.e. will carbon burn in
it? Is it combustible? i.e. does it burn? If so, it unites with
some part of the air. With what part?34. Collecting H by Upward
Displacement.

Experiment 22.--Pass a d.t. from a H generator to the top of a
receiver or t.t. (Fig. 13). The escaping H being so much lighter
than air will force the latter down. To obtain the gas unmixed
with air, the d.t. should tightly fit a cardboard placed under
the mouth of the receiver. When filled, the receiver can be
removed, inverted as usual, and the gas tested. In this and other
experiments for generating H, a thistle-tube, the end of which
dips under the liquid, can be used for pouring in acid, as in
Figure 13.

35. Philosopher's Lamp and Musical Flame.

Experiment 23.--Fit to a cork a piece of glass tubing 10 or 15
cm. long, having the outer end drawn out to a point with a small
opening, and insert it in the H generator. Before igniting the
gas at the end of the tube take the, precaution to collect a t.t.
of it by upward displacement, and bring this in contact with a
flame. If a sharp explosion ensues, air is not wholly expelled
from the generator, and it would be dangerous to light the gas.
When no sound, or very little, follows, light the escaping gas.
The generation of H must not be too rapid, neither should the
t.t. be held under the face, as the cork is liable to be forced
out by the pressure of H. A safety-tube, similar to the thistle-
tube above, will prevent this. This apparatus is called the
"philosopher's lamp." Thrust the flame into a long glass tube 1-
1/2 to 3 cm. in diameter, as shown in Figure 14, and listen for a
musical note.

36. Product of Burning H in Air.

Experiment 24.--Fill a tube 2 or 3 cm. in diameter with calcium
chloride, CaCl2, and connect one end with a generator of H (Fig.
15). At the other end have a philosopher's lamp-tube.Observing
the usual precautions, light the gas and hold over it a receiver,
till quite a quantity of moisture collects. All water was taken
from the gas by the dryer, CaCl2. What is, therefore, the product
of burning H in air? Complete this equation and explain it: 2H +
O = ? Figure 16 shows a drying apparatus arranged to hold CaCl2.

[Fig. 15][Fig. 16]

37. Explosiveness of H.

Experiment 25. -- Fill a soda-water bottle of thick glass with
water, invert it in a pneumatic trough, and collect not over 1/4
full of H. Now remove the bottle, still inverted, letting air in
to fill the other 3/4. Mix the air and H by covering the mouth of
the bottle with the hand, and shaking well; then hold the mouth
of the bottle, slightly inclined, in a flame. Explain the
explosion which follows. If 3/4 was air, what part was O? What
use did the N serve? Note any danger in exploding H mixed with
pure O. What proportions of O and H by volume would be most
dangerously explosive? What proportion by weight?

By the rapid union of the two elements, the high temperature
suddenly expanded the gaseous product, which immediately
contracted; both expansion and contraction produced the noise of
explosion.

38. Pure H Is a Gas without Color, Odor, or Taste.

--It is the lightest of the elements, 14 1/2 times as light
asair. It occurs uncombined in coal-mines, and some other places,
but the readiness with which it unites with other elements,
particularly O, prevents its accumulation in large quantities. It
constitutes two-thirds of the volume of the gases resulting from
the decomposition of water, and one-ninth of the weight. Compute
the latter from its symbol. It is a constituent of plants and
animals, and some rocks. Considering the volume of the ocean, the
total amount of H is large. It can be separated from H2O by
electrolysis, or by C, as in the manufacture of water gas.

When burned with O it forms H2O. Pure O and H when burning give
great heat, but little light. The oxy-hydrogen blow-pipe (Fig.
17) is a device for producing the highest temperatures of
combustion. It has O in the inner tube and H in the outer. Why
would it not be better the other way? These unite at the end, and
are burned, giving great heat. A piece of lime put into the flame
gives the brilliant Drummond or calcium light.