Carbon dioxide (KAR-bun dye-OK-side) is a colorless,
odorless, tasteless, non-combustible gas that can also exist
under pressure as a clear, colorless, odorless, tasteless liquid
and as a white, snow-like solid commonly known as dry ice.
When dry ice is warmed it sublimes (passes directly from
the solid to the gaseous state without first melting) at
78.4C (-109F).
The true nature of carbon dioxide was discovered over an
extended period of time beginning with the research of the
Flemish physician and chemist Jan Baptista van Helmont
(1580–1635?). In about 1603, van Helmont isolated a gas
produced during the combustion of wood and proved that it
was distinct from air. At the time, air was generally regarded
as an element that could not be divided into separate components.
Van Helmont called the gas gas sylvestre (‘‘wood gas’’),
a substance we now know to be carbon dioxide. Credit for
understanding the true nature of carbon dioxide also goes to
the Scottish chemist Joseph Black (1728–1799) who produced
carbon dioxide by heating calcium carbonate (CaCO3). Black
called the gas fixed air and conducted the first extensive
studies of its properties.
The first practical use for carbon dioxide was discovered
in the mid-eighteenth century by the English chemist Joseph
Priestley (1733–1804). Priestley found that passing carbon
dioxide into water produced a sparkling, refreshing drink
that he predicted would one day become a great commercial
success. He was, of course, correct, since water containing
carbon dioxide is the basic component of which all soda
drinks are made.
Carbon dioxide is produced in nature by a number of
reactions. Among the most common is the combustion
(burning) of the fossil fuels (coal, oil, and natural gas). The
gas is also produced during the decay of organic material,
the fermentation of carbohydrates by yeast, and the respiration
of animals. In the laboratory, the simplest and most
direct method of preparation is to treat a carbonate, such
as calcium carbonate, with an acid, such as hydrochloric
acid (HCl).
Carbon dioxide is obtained commercially as the byproduct
of a number of industrial reactions. For example,
when calcium carbonate is heated to produce lime (CaO),
carbon dioxide is released and captured as a by-product. The
steam reforming (refining) of petroleum results in the production
of a mixture of gases known as synthesis gas, consisting
of carbon dioxide, carbon monoxide, hydrogen, and
nitrogen. Carbon dioxide can be separated from the other
components of synthesis gas for commercial uses. Carbon
dioxide also produces as a by-product of the manufacture of
ammonia (NH3) by the Haber-Bosch process.
Carbon dioxide plays an essential role in most biological
processes that take place on Earth’s surface. Plants use carbon
dioxide as a raw material to make the carbohydrates on
which their structures are based. When animals eat plants,
those carbohydrates are then used to build and maintain
their body structures.
In addition to its role in natural processes, carbon dioxide
has many commercial and industrial applications. One of the
most important uses is in the carbonation of beverages.
Although beers and sparkling wines contain carbon dioxide
from natural sources (the fermentation of sugars by
yeasts), nearly all carbonated beverages have their carbon
dioxide added artificially. The carbon dioxide adds a zesty
taste to the beverage and helps to preserve it.
Carbon dioxide is also used as a fire extinguishing agent.
Its use for this purpose is based on the facts that it does not
burn itself and is heavier than air. Thus, when sprayed on a
fire, carbon dioxide settles down on top of the flames and
prevents oxygen from reaching the burning material. The
carbon dioxide can be supplied in a variety of ways in a fire
extinguisher. In some devices, carbon dioxide gas is produced
as the result of a chemical reaction that occurs within
the fire extinguisher. In other devices, liquid carbon dioxide
is released from the extinguisher.
Carbon dioxide is also used in gaseous, liquid, or solid
form as a refrigerant. As a gas, it is used as the ‘‘working
fluid’’ in refrigerators, the fluid that circulates through the
refrigerator changing back and forth from gas to liquid,
absorbing heat in the process. In the form of dry ice, carbon
dioxide is a very efficient and convenient method for cooling
objects to very low temperatures (close to the sublimation
point of carbon dioxide, about 78.4C (109F).
Some other uses of carbon dioxide include the following:
• As an aerosol propellant;
• To provide an oxygen-free atmosphere in which to conduct
welding and other operations with flammable
materials;
• In the industrial manufacture of carbonates;
• For cloud seeding to promote modifications in the
weather (increases or decreases in rain fall);
• In the fumigation of rice to preserve the product for
extended periods of time;
• As an artificial smoke in theater productions;
• As a moderator to slow down the speed of neutrons
traveling in a nuclear power plant;
• In the frozen food industry;
• To enrich the air in a greenhouse, providing additional
carbon dioxide to promote plant growth; and
• For the hardening of foundry molds and cores.
In general, carbon dioxide poses little or not threat to
humans in concentrations to which one is normally exposed.
Dry ice may pose a hazard if not handled carefully as its very
low temperature can cause damage to the skin.
