CHAPTER VI. OXYGEN.

22. To Obtain Oxygen.

Experiment 13.--Take 5 g. of crystals of potassium chlorate
(KClO3) and, without pulverizing, mix with the same weight of
pure powdered manganese dioxide (MnO2). Put the mixture into a
t.t., and insert a d.t.--delivery-tube--having the cork fit
tightly. Hang it on a r.s.--ring-stand,-- as in Figure 7, having
the other end of the d.t.

(Fig 7.)

under the shelf, in a pneumatic trough, filled with water just
above the shelf. Fill three or more receivers--wide-mouthed
bottles--with water, cover the mouth of each with a glass plate,
invert it with its mouth under water, and put it on the shelf of
the trough, removing the plate. No air should be in the bottles.
Have the end of the d.t. so that the gas will rise through the
orifice. Hold a lighted lamp in the hand, and bring the flame
against the mixture in the t.t. Keep

the lamp slightly in motion, with the hand, so as not to break
the t.t. by over-heating in one place. Heat the mixture strongly,
if necessary. The upper part of the t.t. is filled with air:
allow this to escape for a few seconds; then move a receiver over
the orifice, and fill it with gas. As soon as the lamp is taken
away, remove the d.t. from the water. The gas contracts, on
cooling, and if not removed, water will be drawn over, and the
t.t. will be broken. Let the t.t. hang on the r.s. till cool.

With glass plates take out the receivers, leaving them covered,
mouth upward (Fig. 8), with little or no water inside. When cool,
the t.t. may be cleaned with water, by covering its mouth with
the thumb or hand, and shaking it vigorously.

What elements, and how many, in KClO3? In Mn02? It is evident
that each of these compounds contains O. Why, then, could we not
have taken either separately, instead of mixing the two? This
could have been done at a sufficiently high temperature. Mu02
requires a much higher temperature for dissociation, i.e.
separation into its elements, than KClO3, while a mixture of the
two causes O to come off from KClO3 at a lower temperature than
if alone. It is not known that Mn02 suffers any change.

Each molecule of potassium chlorate undergoes the following
change:--


Potassium Chlorate = Potassium Chloride + Oxygen
KClO3 = KCl + 3 O.


Is this analysis or synthesis? Complete the equation, by using
weights, and explain it. Notice whether the right- hand member of
the equation has the same number of atoms as the left. Has
anything been lost or gained? What element has heat separated?
Does the experiment show whether O is very soluble in water? How
many grams of O are obtainable from 122.58 g. KCIO3? PROPERTIES.

23. Combustion of Carbon.

OXYGEN Experiment 14.--Examine the gas in one of the receivers.
Put a lighted splinter into the receiver, sliding along the glass
cover. Remove it, blow it out, and put in again while glowing. Is
it re-kindled? Repeat till it will no longer burn. Is the gas a
supporter of combustion? How did the combustion compare with that
in air? Is it probable that air is pure O? Why did the flame at
last go out? Has the O been destroyed, or chemically united with
something else?

Wood is in part C. CO2 is formed by the combustion; name it. The
equation is C + 2O = CO2. Affix the names and weights. Is CO2 a
supporter of combustion? Note that when C is burned with plenty
of O, CO2 is always formed, and that no matter how great the
conflagration, the union is atom by atom. Combustion, as here
shown, is only a rapid union of O with some other substance, as C
or H.

24. Combustion of Sulphur.

Experiment 15.--Hollow out one end of a piece of electric-light
pencil, or of crayon, 3 cm. long, and attach it to a Cu wire
(Fig. 9). Put into this a piece of S as large as a pea, ignite it
by holding in the flame, and then hold it in a receiver of O.
Note the color and brightness of the flame, and compare with the
same in the air. Also note the color and odor of the product. The
new gas is SO2. Name it, and write the equation for its
production from S and O. How do you almost daily perform a
similar experiment? Is the product a supporter of combustion?

25. Combustion of Phosphorus.

Experiment 16.--With forceps, which should always be used in
handling this element, put a bit of P, half as large as the S
above,into the crayon, called a deflagrating-spoon. Heat another
wire, touch it to the P, and at once lower the latter into a
receiver of O. Notice the combustion, the color of the flame and
of the product. After removing, be sure to burn every bit of P by
holding it in a flame, as it is liable to take fire if left. The
product of the combustion is a union of what two elements? Is it
an oxide? Its symbol is P2O5. Write the equation, using symbols,
names, and weights. Towards the close of the experiment, when the
O is nearly all combined, P2O3 is formed, as it is also when P
oxidizes at a low temperature. Name it and write the equation.

26. Combustion of Iron.

Experiment 17.--Take in the forceps a piece of iron picture-cord
wire 6 or 8cm long, hold one end in the flame for an instant,
then dip it into some S. Enough S will adhere to be set on fire
by holding it in the flame again. Then at once dip it into a
receiver of O with a little water in the bottom. The iron will
burn with scintillations. Is this analysis or synthesis? What
elements combine? A watch-spring, heated to take out the temper,
may be used, but picture-wire is better.

The product is Fe3O4. Write the equation. How much Fe by weight
in the formula? How much O? What per cent by weight of Fe in the
compound? Multiply the fractional part by 100. What per cent of
0? Whatper cent of C0 .is C? O2? Find the percentage composition
of SO2. P2O5.

From the last five experiments what do you infer of the tendency
of O to unite with other elements?

27. Oxygen is a Gas without Color, Odor, or Taste.

It is chemically a very active element; that is, it unites with
almost everything. Fluorine is the only element with which it
will not combine. When oxygen combines with a single element,
what is the compound called? We have found that O makes up a
certain portion of the air; later, we shall see how large the
proportion is. Its tendency to combine with almost everything is
a reason for the decay, rust, and oxidation of so many
substances, and for conflagrations, great and small. New
compounds are thusformed, of which O constitutes one factor.
Water, H2O, is only a chemical union of O and H. Iron rust, Fe2O3
and H2O, is composed of O, Fe, and water. The burning of wood or
of coal gives rise to carbon dioxide, CO2, and water. Decay of
animal and vegetable matter is hastened by this all-pervading
element. O forms a portion of all animal and vegetable matter, of
almost all rocks and minerals, and of water. It is the most
abundant of all elements, and makes up from one-half to two-
thirds of the earth's surface. Compute the proportion of it, by
weight, in water, H2O. It is the union of O in the air with C and
H in our blood that keeps up the heat of the body and supports
life. See page 81.

There are many ways of preparing this element besides the one
given above. It may be obtained from water (Experiment 38) and
from many other compounds, e.g. by heating mercury oxide,
HgO.