SI units were used
where possible. Unless otherwise stated, standard conditions were used.
Disclaimer and references
Carbon dioxide is an atmospheric gas composed of one carbon
and two oxygen atoms. One of the best known of chemical compounds, it is
frequently called by its formula:
CO2 (see oh two)
People and animals produce carbon dioxide when they breathe. If you hold
your breath too long, it starts to hurt, because the lungs get too full of
carbon dioxide.
Plants thrive on carbon dioxide, absorbing the carbon and releasing oxygen
for us and our pets to breathe.
Carbon dioxide results from the combustion of organic matter if sufficient
amounts of oxygen are present. It is also produced by various microorganisms
in fermentation and cellular respiration. Plants utilize carbon dioxide during
photosynthesis, using both the carbon and the oxygen to construct carbohydrates.
It is present in the Earth's atmosphere at a low concentration and acts as
a greenhouse gas. It is a major component of the carbon cycle.
Table of contents
1 Chemical and physical properties
2 Uses
3 Biology
4 Atmosphere
4.1 Atmospheres of other planets
5 Oceans
6 History
7 External links
Chemical and physical properties
Carbon dioxide is a colorless gas which, when breathed in high concentrations
(a dangerous activity due to the associated asphyxiation risk), produces
a slightly sour taste in the mouth and stinging sensation in the nose and
throat. These effects are due to the fact that the gas is dissolving in the
mucous membranes and saliva, forming a weak solution of carbonic acid. Its
density at 298K is 1.98 kg m-3, 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.
Carbon dioxide can be reduced to a liquid and solid form by intense pressure.
At standard pressure, it is never liquid: it directly passes between the
gaseous and solid phase at -78°C; in a process called sublimation.
Water will absorb its own volume of carbon dioxide, and more than this under
pressure. About 1% of the dissolved carbon dioxide turns into carbonic acid.
The carbonic acid in turn dissociates partly to form bicarbonate and carbonate
ions.
In energetic atmospheres, such as those present in a welding arc, Carbon
Dioxide acts as an oxidizer of most metals. Use in the automotive industry
is common despite significant evidence that it causes brittleness of the
weld joint and such joints deteriorate over time due to the formation of
carbonic acid. It is used as a welding gas primarily because it is much less
expensive than argon or helium.
Uses
Carbon dioxide in its solid frozen form it is also known as dry
ice. It is used
for cooling
to produce 'dry ice fog' for special effects: when dry ice is put into contact
with water, the resulting mixture of CO2 and cold humid air causes
condensation and a fog
for cleaning: shooting tiny dry ice pellets at a surface cools the dirt and
causes it to pop off
Dry ice is produced by compressing CO2 to a liquid form, removing
excess heat, and then letting the liquid carbon dioxide expand quickly. This
expansion causes a drop in temperature so that some of the CO2
freezes to "snow" which is then compressed.
Carbon dioxide extinguishes flames, and some fire extinguishers contain pressured
liquid carbon dioxide. Life jackets often contain capsules of pressured liquid
carbon dioxide used for quick inflation.
Water containing dissolved carbon dioxide is also known as carbonated
water or soda water. Carbonated water is contained in many soft
drinks and some natural springs. Some beverages, such as beer and sparkling
wine contain carbon dioxide as a result of fermentation.
Many leavening agents used for baking produce carbon dioxide to cause the
dough to rise. Examples are baker's yeast and baking powder.
Carbon Dioxide is the primary gas respirated by plants. Greenhouses use
CO2 enrichment to boost plant growth.
High level of carbon dioxide in the atmosphere effectively exterminate most
pests. Greenhouses will raise the level of CO2 to 10,000 ppm (1%)
for several hours to eliminate pests such as whitefly, spider mites, and
others.
Biology
Carbon dioxide is a waste product in organisms that obtain energy from breaking
down sugars or fats with oxygen as part of their metabolism, in a process
known as cellular respiration. This includes all animals, many fungi and
some bacteria. In higher animals, the carbon dioxide travels in the blood
(where most of it is held in solution) from the body's tissues to the lungs
where it is exhaled.
Carbon dioxide content in fresh air is less than 1%, in exhaled air ca. 4.5%.
When breathed in in high concentrations (about 5% by volume), it is toxic
to humans and other animals. Hemoglobin, the main molecule in red blood cells,
can bind both to oxygen and to carbon dioxide. If the CO2
concentration is too high, then all hemoglobin is saturated with carbon dioxide
and no oxygen transport takes place (even if plenty of oxygen is in the air).
As a result, people in a poorly ventilated room will experience difficulty
breathing due to accumulated carbon dioxide, even before lack of oxygen becomes
a problem. Carbon dioxide, either as a gas or as dry ice, should be handled
only in well ventilated areas.
OSHA limits carbon dioxide concentration in the workplace to 0.5% for prolonged
periods, or to 3% for brief exposures (up to ten minutes). OSHA considers
concentrations exceeding 4% as "immediately dangerous to life and health."
People who breathe 5% carbon dioxide for more than half an hour show signs
of intoxication, while breathing 7-10% carbon dioxide can produce unconsciousness
in only a few minutes.
The CO2 that is carried in blood can be find in different areas.
8% of CO2 is in the plasma as a gas. 20% of it is bound to hemoglobin,
The CO2 bounded to hemoglobin is not competing with oxygen binding
since it binds to amino acids rather than hemo molecules.The remaining 72%
of it is carried as bicarbonate HCO3- which is a buffer important
in our pH regulation. The level of bicarbonate is regulated and if it is
high then we breath more rapidly to get rid of the excess carbon dioxide.
The level of carbon dioxide/bicarbonate in the blood affects the thickness
of the blood capillaries. If it is high, the capillaries expand and more
blood rushes in and carries the excess bicarbonate to the lungs. To help
avoid the loss of carbon dioxide to a deadly low level, the body has developed
certain defensive mechanisms. These include contractions of the air pipes
and blood pipes, and the increased production of mucus.
Plants remove carbon dioxide from the atmosphere by photosynthesis, which
uses light energy to produce organic plant materials by combining carbon
dioxide and water. This releases free oxygen gas. Sometimes carbon dioxide
gas is pumped into greenhouses to promote plant growth.
Atmosphere
experiment done by the Monterey Bay Aquarium Research Institute.]]
As of 2004, the earth's atmosphere is about 0.038% CO2 by volume,
or 379 ppm. Due to the greater land area, and therefore greater plant life,
in the northern hemisphere as compared to the southern hemisphere, there
is an annual fluctuation of about 5 ppm, peaking in May and reaching a minimum
in October at the end of the northern hemisphere growing season, when the
quantity of biomass on the planet is greatest.
Despite its small concentration, CO2 is a very important component
of Earth's atmosphere, because it traps infrared radiation and enhances the
greenhouse effect of water vapor, thus keeping the Earth from cooling down.
The initial carbon dioxide in the atmosphere of the young Earth was produced
by volcanic activity; this was necessary for a warm and stable climate conducive
to life. Volcanic activity now releases about 130-230 million metric tons
(145-255 million short tons) of carbon dioxide each year. Volcanic releases
are about 1% the amount which is released by human activities.
As of 2004, atmospheric CO2 has increased about 35 percent since
the start of the Industrial Revolution, with an increase of 20 percent since
1958, based on measurements taken at Mauna Loa. Burning fossil fuels such
as coal and petroleum is the leading cause of increased man-made
CO2; deforestation the second major cause.
The global warming hypothesis was first recorded in scientific literature
near the end of the 19th century. It predicts that increased amounts of
CO2 in the atmosphere tend to increase the greenhouse effect and
thus contribute to global warming. The size of this effect is still a matter
of debate. The widespread opinion that there is currently a warming phase
and that the increased carbon dioxide concentration is a major contributor
to it has led to international agreements such as the Kyoto Protocol which
aim to regulate the release of CO2 into the atmosphere.
See also: global warming controversy.
Atmospheres of other planets
Carbon dioxide is the main component of the atmospheres of Mars and Venus.
Oceans
The Earth's oceans dissolve a major amount of carbon dioxide. The resulting
carbonate anions bind to cations present in sea water such as Ca2+
and Mg2+ to form deposits of limestone and dolomite. Most carbon
dioxide in the atmosphere eventually undergoes this fate: if all the carbonate
rocks in the earth's crust were to be converted back in to carbon dioxide,
the resulting carbon dioxide would weigh 40 times as much as the rest of
the atmosphere.
History
Carbon dioxide was first described by the chemist Jan Baptist van Helmont
in the 17th century.
External links
Dry Ice information
Bassam Z. Shakhashiri: Chemical of the Week: Carbon Dioxide
Keeling, C.D. and T.P. Whorf: Atmospheric carbon dioxide record from Mauna
Loa, 2002
Mauna Loa 2004 update
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