182 GROUP IVthe bottom of towers or wells in which it is being prepared, and may
reach dangerous concentrations there. (Carbon dioxide does not
support respiration, but it is not toxic.) Its critical point is 304 K.
i.e. it may be compressed to a liquid below this temperature. How-
ever, if carbon dioxide is cooled rapidly (for example by allowing
compressed gas to escape through a valve) solid carbon dioxide is
formed. This sublimes at 195 K and atmospheric pressure; it is a
white solid, now much used as a refrigerant (known as kdry ice' or
^DrikokT), since it leaves no residue after sublimation.
Chemically, carbon dioxide is not very reactive, and it is often used
as an inactive gas to replace air when the latter might interact with
a substance, for example in the preparation of chromium(II) salts
(p. 383). Very reactive metals, for example the alkali metals and
magnesium can, however, continue to burn in carbon dioxide if
heated sufficiently, for example
4K 4- 3CO 2 -> 2K 2 CO 3 + C
Carbon dioxide reacts with a solution of a metal hydroxide giving
the carbonate, which may be precipitated, for example
Ca2+ + 2OH~ + CO 2 -> CaCO 3 i + H 2 O
This reaction is used as a test for carbon dioxide. Passage of an excess
of carbon dioxide produces the soluble hydrogencarbonate :
CaCO 3 + CO 2 + H 2 O -» Ca2+ + 2HCOJ
The hydrogencarbonate ion, produced in nature by this reaction, is
one of the main causes of temporary hardness in water. Carbon
dioxide is fairly soluble in water, 1 cm^3 dissolving 1.7 cm^3 of the gas
at stp. The variation of solubility with pressure does not obey
Henry's law, since the reaction
CO 2 + H 2 O^=^~
takes place to a small extent, forming carbonic acid (see below).USES
Carbon dioxide is used in the manufacture of sodium carbonate by
the ammonia-soda process, urea, salicyclic acid (for aspirin), fire
extinguishers and aerated water. Lesser amounts are used to transfer
heat generated by an atomic reactor to water and so produce steam
and electric power, whilst solid carbon dioxide is used as a refrigerant,
a mixture of solid carbon dioxide and alcohol providing a good
low-temperature bath (195 K) in which reactions can be carried out
in the laboratory.