Carbon Dioxide
Carbon dioxide is an atmospheric
gas composed of one carbon and two oxygen atoms. A very widely known
chemical compound, it is frequently called by its formula CO2.
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Carbon dioxide results from the combustion of organic matter if
sufficient amounts of oxygen are present. It is also produced by
various microorganisms from fermentation and cellular respiration.
Plants utilize carbon dioxide during photosynthesis, using both
the carbon and the oxygen to construct carbohydrates. Plants also
release oxygen to the atmosphere which is subsequently used by animals,
many fungi and some bacteria for respiration (breathing).
To test for this gas. When a lighted splint
is inserted into a test tube containing this gas, it is immediately
extinguished, as carbon dioxide does not support combustion.
Chemical and Physical Properties
Carbon dioxide is a colorless gas which, when inhaled at high concentrations,
produces a sour taste in the mouth and stinging sensation in the
nose and throat. Carbon dioxide, either as a gas or as dry
ice, should be handled only in well ventilated areas.
Its density is about 1.5 times that of air. The carbon dioxide
molecule (O=C=O) contains two double bonds and has a linear shape.
It has no electrical dipole. As it is fully oxidized, it is not
very reactive and in particular not flammable.
At temperatures below -78oC (-109oF), carbon dioxide condenses
into a white solid called dry ice. Liquid
carbon dioxide forms only at pressures above 5.1 atm; at atmospheric
pressure, it passes directly between the gaseous and solid phases
in a process called sublimation.
Uses
Carbon dioxide is used to produce carbonated soft drinks and soda
water. Traditionally, the carbonation in beer and sparkling wine
comes about through natural fermentation, but some manufacturers
carbonate these beverages artificially using carbon dioxide.
Liquid and solid carbon dioxide are important refrigerants, especially
in the food industry, where they are employed during the transportation
and storage of frozen foods, and in the medical field, where they
are used for transportation and preservation of laboratory specimens.
In medicine, up to 5% of carbon dioxide is added to pure oxygen
used in medicine for stimulation of breathing after apnea and to
stabilize the O2/CO2 balance in blood.
Carbon dioxide is often used an inexpensive, non-flammable pressurized
gas, used to inflate life jackets. Steel capsules are also
sold as supplies of compressed gas for airguns, paintball markets
and for making seltzer. Carbon dioxide extinguishes flames,
and some fire extinguishers, especially those designed for electrical
fires, contain liquid carbon dioxide under pressure.
Liquid carbon dioxide is a good solvent for many organic compounds,
and is used to remove caffeine from coffee. It has begun to attract
attention in the pharmaceutical and other chemical processing industries
as a less toxic alternative to more traditional solvents such as
organocholorides. Carbon dioxide is used as a medium in a
common type of industrial gas laser known as the carbon dioxide
laser.
Greenhouses may enrich their atmospheres with additional CO2 to
boost plant growth while eliminating pests such as whitefly, spider
mites and others. Proposals have been made that carbon dioxide from
power generation could be added into ponds to grow algae that could
then be converted into biodiesel fuel.
Dry ice is used in cleaning:
shooting tiny dry ice pellets at a surface cools the dirt and causes
it to pop off. This technique is rapidly becoming popular for cleaning
printing presses, since the dirt and print matter falls to the ground
and can easily be disposed, while there is no solvent, cleaner or
water residue to affect the equipment as the dry ice pellets completely
dissolve into the atmosphere.
Dry ice is used in theaters to produce
fog as a special effect. It is commonly injected into
or adjacent to producing oil wells to act as a pressurizing agent.
When dissolved into the underground crude oil, the carbon dioxide
will significantly reduce the viscosity of the oil enabling the
oil to flow more rapidly through the earch to the removal well.
In mature oil fields, extensive pipe networks are used to carry
the carbon dioxide to the injection points.
Oceans
The Earth’s oceans contain a huge amount of carbon dioxide
in the form of bicarbonate and carbonate ions—much more than
the amount in the atmosphere. The bicarbonate is produced in reactions
between rock, water, and carbon dioxide. One example is the dissolution
of calcium carbonate:
CaCO3 + CO2 + H2O Ca2+ + 2 HCO3-
These reactions tend to buffer changes in atmospheric CO2. Reactions
between carbon dioxide and non-carbonate rocks also add bicarbonate
to the seas, which can undergo the reverse of the above reaction
to form carbonate rocks, releasing half of the bicarbonate as CO2.
Over hundreds of millions of years this has produced huge quantities
of carbonate rocks. If all the carbonate rocks in the Earth’s
crust were converted back into carbon dioxide, the resulting carbon
dioxide would weigh 40 times as much as the rest of the atmosphere.
The vast majority of CO2 added to the atmosphere will eventually
be absorbed by the oceans and become bicarbonate ion, but the process
takes on the order of a hundred years because most seawater rarely
comes near the surface.
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