1 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.

2 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. Steel capsules are also sold as supplies of compressed gas in airguns, paintball markers and BB guns.

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 one of the main raw materials (with water) in the photosynthesis process. Greenhouses may use low levels of 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.

A common type of industrial laser uses carbon dioxide as a medium.

3 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 plants, 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 acute hypercapnia, while breathing 7–10% carbon dioxide can produce unconsciousness in only a few minutes.

The CO₂ that is carried in blood can be found 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. Plants also emit CO₂ during respiration; but on balance they are net sinks of CO₂.

4 Atmosphere

An image of liquid carbon dioxide spilling from a beaker on the ocean floor. The image was taken from video of a carbon sequestration 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.

5 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^-

Reactions like this tend to buffer changes in atmospheric CO₂. Reactions between carbon dioxide and non-carbonate rocks also add bicarbonate to the seas, which can later 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 to be 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.

6 History

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

7 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

Oxides Greenhouse gases Propellants

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