The Carbon dioxide reference article from the English Wikipedia on 24-Jul-2004
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Carbon dioxide

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kJ/mol >
General
Name Carbon dioxide
Chemical Formula CO2
Appearance Colourless gas
Physical
Formula weight amu
Melting point 216 K (-57 °C)
Boiling point 195 K (-78 °C)
Density 3 kilogram>kg/m3 (solid)
1.98 kg/m3 (gas at 298 K)
Solubility g in 100g water
Thermochemistry
ΔfH0gas
ΔfH0solid ? kJ/mol
S0gas, 1 bar 213.79 J/mol·K
S0solid ? J/mol·K
Safety
Ingestion May cause nausea, vomiting, GI hemorrhage.
Inhalation Asphyxiant (suffocating), causes hyperventilation. Repeated exposure dangerous.
Skin Dry ice may cause frostbite.
Eyes Can be dangerous.
More info Hazardous Chemical Database
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
5 Oceans
6 History
7 External links

Chemical and physical properties

Carbon dioxide is a colorless gas with a weak odor. 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, resulting in a slightly acidic taste. 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

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

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