discussed in
a previous chapter. Water highly charged with carbon dioxide is used for
making soda water and similar beverages. Since it is a non-supporter of
combustion and can be generated readily, carbon dioxide is also used as
a fire extinguisher. Some of the portable fire extinguishers are simply
devices for generating large amounts of the gas. It is not necessary
that all the oxygen should be kept away from the fire in order to
smother it. A burning candle is extinguished in air which contains only
2.5% of carbon dioxide.

~Carbonic acid~ (H_{2}CO_{3}). Like most of the oxides of the non-metallic
elements, carbon dioxide is an acid anhydride. It combines with water to
form an acid of the formula H_{2}CO_{3}, called carbonic acid:

H_{2}O + CO_{2} = H_{2}CO_{3}.

The acid is, however, very unstable and cannot be isolated. Only a very
small amount of it is actually formed when carbon dioxide is passed into
water, as is evident from the small solubility of the gas. If, however,
a base is present in the water, salts of carbonic acid are formed, and
these are quite stable:

2NaOH + H_{2}O + CO_{2} = Na_{2}CO_{3} + 2H_{2}O.

~Action of carbon dioxide on bases.~ This conduct is explained by the
principles of reversible reactions. The equation

H_{2}O +CO_{2} <--> H_{2}CO_{3}

is a reversible equation, and the extent to which the reaction
progresses depends upon the relative concentrations of each of the three
factors in it. Equilibrium is ordinarily reached when very little
H_{2}CO_{3} is formed. If a base is present in the water to combine with
the H_{2}CO_{3} as fast as it is formed, all of the CO_{2} is converted
into H_{2}CO_{3}, and thence into a carbonate.

~Salts of carbonic acid,--carbonates.~ The carbonates form a very
important class of salts. They are found in large quantities in nature,
and are often used in chemical processes. Only the carbonates of sodium,
potassium, and ammonium are soluble, and these can be made by the action
of carbon dioxide on solutions of the bases, as has just been explained.

The insoluble carbonates are formed as precipitates when soluble salts
are treated with a solution of a soluble carbonate. Thus the insoluble
calcium carbonate can be made by bringing together solutions of calcium
chloride and sodium carbonate:

CaCl_{2} + Na_{2}CO_{3} = CaCO_{3} + 2NaCl.

Most of the carbonates are decomposed by heat, yielding an oxide of the
metal and carbon dioxid