Carbon dioxide


Name	Carbon dioxide
Chemical Formula	C O₂
Appearance	Colourless gas
Physical
Formula weight	44.0 amu
Melting point	Liquifies under high pressure at 216 K (-57 °C)
Boiling point	sublimes at 195 K (-78 °C)
Density	1.6 ×10³ kg/m³ (solid) 1.98 kg/m³ (gas at 298 K)
Solubility	0.145 g in 100g water
Thermochemistry
Δ_fH⁰_gas	-393.52 kJ/mol
Δ_fH⁰_solid	? kJ/mol
S⁰_{gas, 1 bar}	213.79 J/mol·K
S⁰_solid	? 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

The chemical compound carbon dioxide, or CO₂, is an atmospheric gas composed of one carbon and two oxygen atoms. 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 is breathed out by animals. Plants absorb 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.

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.

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, when breathed in high concentrations (about 5% by volume), 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). Carbon dioxide and dry ice should therefore only be handled in well ventilated areas.

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 hemolobin 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

The earth's atmosphere contains about 0.037% or 370 ppm CO₂ by volume. 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.

Atmospheric CO₂ has increased about 30 percent since the early 1800s, with an estimated increase of 17 percent since 1958. 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.

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 Belgian 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\n