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.

Carbon dioxide is a colorless gas with a weak odor. It is about 1.5 times as dense as air. The carbon dioxide molecule
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.


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


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