Carbon dioxide

Properties

Name	Carbon dioxide
Chemical Formula	CO2
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)
Solubility	0.145 g in 100g water
Thermochemistry
ΔfH0gas	-393.52 kJ/mol
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 (http://ull.chemistry.uakron.edu/erd/chemicals/7/6259)
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 sufficien amounts of oxygen are present. It is also produced by various microorganisms in fermentation and is breathed out by animals. Plants take it in for their nutrition and growth, the carbon being retained and the oxygen released. 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. It is about 1.5 times as dense as 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.

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

Despite its low concentration of about 0.037% or 370 ppm by volume, CO₂ is a very important component of Earth's atmosphere, because it traps infrared radiation and thus participates in the greenhouse effect. 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 CO₂, deforestation the second major cause).

The increased amounts of CO₂ in the atmosphere are widely believed to enhance the greenhouse effect and thus contribute to global warming; this has led to international agreements such as the Kyoto Protocol which aim to limit the release of CO₂ into the atmosphere. Some have proposed carbon sequestration as a method to reduce the concentration (or at least slow the rate of increase) of human-made CO₂ from entering the atmosphere.

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.

History

Carbon dioxide was first described in the 17th century by the Belgian chemist Jan Baptist van Helmont[?].

External links

• Dry Ice information (http://www.dryiceinfo.com/science.htm)
• Bassam Z. Shakhashiri: Chemical of the Week: Carbon Dioxide (http://scifun.chem.wisc.edu/chemweek/CO2/CO2)
• Keeling, C.D. and T.P. Whorf: Atmospheric carbon dioxide record from Mauna Loa (http://cdiac.esd.ornl.gov/trends/co2/sio-mlo.htm), 2002