CHAPTER XXV. CARBON DIOXIDE.

120. Preparation.

Experiment 74.--Put into a t.t., or a bottle with a d.t. and a
thistle-tube, 10 or 20 g. CaCO3, marble in lumps; add as many
cubic centimeters of H2O, and half as much HCl, and collect the
gas by downward displacement (Fig. 39). Add more acid as needed.
CaCO3 + 2 HCl = CaCl2 + H2CO3. H2CO3 = H2O + CO2. H2CO3 is a very
weak compound, and at once breaks up. By some, its existence as a
compound is doubted.

121. Tests.

Experiment 75.--(1) Put a burning and a glowing stick into the
t.t. or bottle. (2) Hold the end of the d.t. directly against the
flame of a small burning stick. Does the gas support combustion?
(3) Pour a receiver of the gas over a candle flame. What does
this show of the weight of the gas? (4) Pass a little CO2 into
some H2O (Fig. 32), and test it with litmus. Give the reaction
for the solution of CO2 in H2O.

Experiment 76.--Put into a t.t. 51 cc. of clear Ca(OH)2 solution,
i.e. lime water; insert in this the end of a d.t. from a CO2
generator (Fig. 32). Notice any ppt. formed. It is CaCO3. Let the
action continue until the ppt. disappears and the liquid is
clear. Then remove the d.t., boil the clear liquid for a minute,
and notice whether the ppt. reappears.

122. Explanation.

Ca(OH)2 + CO2 = CaCO3 + H2O. The curious phenomena of this
experiment are explained by the solubility of CaCO3 in water
containing CO2, and its insolu-bility in water, having no CO2.
When all the Ca(OH)3 is combined, or changed to CaCO3, the excess
of CO2 unites with H2O, forming the weak acid H2CO3, which
dissolves the precipitate, CaCO3, and gives a clear liquid. On
heating this, H2CO3 gives up its CO2, and CaCO3 is
reprecipitated, not being soluble in pure water.

Lime water, Ca(OH)2 solution, is therefore a test for the
presence of CO2. To show that carbon dioxide is formed in
breathing, and in the combustion of C, and that it is present in
the air, perform the following experiment:

Experiment 77.--(1) Put a little lime water into a t.t., and blow
into it through a piece of glass tubing. Any turbidity shows
what? (2) Burn a candle for a few minutes in a receiver of air,
then take out the candle and shake up lime water with the gas.
(3) Expose some lime water in an e.d. to the air for some time.

133. Oxidation in the Human System.--Carbon dioxide, or carbonic
anhydride, carbonic acid, etc., CO2, is a heavy gas, without
color or odor. It has a sharp, prickly taste, and is commonly
reckoned as poisonous if inhaled in large quantities, though it
does not chemically combine with the blood as CO does. Ten per
cent in the air will sometimes produce death, and five per cent
produces drowsiness. It exists in minute portions in the
atmosphere, and often accumulates at the bottom of old wells and
caverns, owing to its slow diffusive power. Before going down
into one of these, the air should always be tested by lowering a
lighted candle. If this is extinguished, there is danger. CO2 is
the deadly "choke damp" after a mine explosion, CH4 being
converted into CO2 and H2O; a great deal is liberated during
volcanic eruptions, and it is formed in breathing by the union of
O in the air with C in the system. This union of C and O takes
place in the lungs and in all the tissues of the body, even on
the surface. Oxygen is taken into the lungs, passes through the
thin membrane into the blood, forms a weak chemical union with
the red corpuscles, and is conveyed by them to all parts of the
system. Throughout the body, wherever necessary, C and H are
supplied for the O, and unite with it to form CO2 and H2O. These
are taken up by the blood though they do not form a chemical
union with it, are carried to the lungs, and pass out, together
with the unused N and surplus O. The system is thus purified, and
the waste must be supplied by food. The process also keeps up the
heat of the body as really as the combustion of C or P in O
produces heat. The temperature of the body does not vary much
from 99 degrees F., any excess of heat passing off through
perspiration, and being changed into other forms of energy.

If, as in some fevers, the temperature rises above about 105
degrees F., the blood corpuscles are killed, and the person dies.
During violent exercise much material is consumed, circulation is
rapid, and quick breathing ensues. Oxygen is necessary for life.
A healthy person inhales plentifully; and this element is one of
nature's best remedies for disease. Deep and continued
inhalations in cold weather are better than furnace fires to heat
the system. All animals breathe O and exhale CO2. Fishes and
other aquatic animals obtain it, not by decomposing H2O, but from
air dissolved in water. Being cold-blooded, they need relatively
little; but if no fresh water is supplied to those in captivity,
they soon die of O starvation.

124. Oxidation in Water.--Swift-running streams are clear and
comparatively pure, because their organic impurities are
constantly brought to the surface and oxidized, whereas in
stagnant pools these impurities accumulate. Reservoirs of water
for city supply have sometimes been freed from impurities by
aeration, i.e. by forcing air into the water.

125. Deoxidation in Plants.--Since CO2 is so constantly poured
into the atmosphere, why does it not accumulate there in large
quantity? Why is there not less free O in the air to-day than
there was a thousand years ago? The answer to these questions is
found in the growth of vegetation. In the leaf of every plant are
thousands of little chemical laboratories; CO2 diffused in small
quantities in the air passes, together with a very little H2O,
into the leaf, usually from its under side, and is decomposed by
the radiant energy of the sun. The C is built into the woody
fiber of the tree, and the O is ready to be re-breathed or burned
again. CO2 contributes to the growth of plants, O to that of
animals; and the constituents of the atmosphere vary little from
one age to another. The compensation of nature is here well
shown. Plants feed upon what animals discard, transforming it
into material for the sustenance of the latter, while animals
prepare food for plants. All the C in plants is supposed to come
from the CO2 in the atmosphere. Animals obtain their supply from
plants. The utility of the small percentage of CO2 in the air is
thus seen.

126. Uses.--CO2 is used in making "soda-water," and in chemical
engines to put out fires in their early stages. In either case it
may be prepared by treating Na2CO3 or CaCO3 with H2SO4. Give the
reactions. On a small scale CO2 is made from HNaCO3. CO2 has a
very weak affinity for water, but probably forms with it H2CO3.
Much carbon dioxide can be forced into water under pressure. This
forms soda-water, which really contains no soda. The
justification for the name is the material from which it is
sometimes made. Salts from H2CO3, called carbonates, are
numerous, Na2CO3 and CaCO3 being the most important.