The complete reaction is represented by the equation

CaCO_{3} + 2HCl = CaCl_{2} + CO_{2} + H_{2}O.

~Physical properties.~ Carbon dioxide is a colorless, practically odorless
gas whose density is 1.5. Its weight may be inferred from the fact that
it can be siphoned, or poured like water, from one vessel downward into
another. At 15 deg. and under ordinary pressure it dissolves in its own
volume of water and imparts a somewhat biting, pungent taste to it. It
is easily condensed, and is now prepared commercially in this form by
pumping the gas into steel cylinders (see Fig. 6) which are kept cold
during the process. When the liquid is permitted to escape into the air
part of it instantly evaporates, and in so doing absorbs so much heat
that another portion is solidified, the solid form strikingly resembling
snow in appearance. This snow is very cold and mercury can easily be
frozen with it.

~Solid carbon dioxide.~ Cylinders of liquid carbon dioxide are
inexpensive, and should be available in every school. To demonstrate the
properties of solid carbon dioxide, the cylinder should be placed across
the table and supported in such a way that the stopcock end is several
inches lower than the other end. A loose bag is made by holding the
corners of a handkerchief around the neck of the stopcock, and the cock
is then turned on so that the gas rushes out in large quantities. Very
quickly a considerable quantity of the snow collects in the
handkerchief. To freeze mercury, press a piece of filter paper into a
small evaporating dish and pour the mercury upon it. Coil a flat spiral
upon the end of a wire, and dip the spiral into the mercury. Place a
quantity of solid carbon dioxide upon the mercury and pour 10 cc.-15 cc.
of ether over it. In a minute or two the mercury will solidify and may
be removed from the dish by the wire serving as a handle. The filter
paper is to prevent the mercury from sticking to the dish; the ether
dissolves the solid carbon dioxide and promotes its rapid conversion
into gas.

~Chemical properties.~ Carbon dioxide is incombustible, since it is, like
water, a product of combustion. It does not support combustion, as does
nitrogen peroxide, because the oxygen in it is held in very firm
chemical union with the carbon. Very strong reducing agents, such as
highly heated carbon, can take away half of its oxygen:

CO_{2} + C = 2CO.

~Uses.~ The relation of carbon dioxide to plant life has been