Carbon dioxide (chemical formula CO2) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature, and as the source of available carbon in the carbon cycle, atmospheric CO2 is the primary carbon source for life on Earth. In the air, carbon dioxide is transparent to visible light but absorbs infrared radiation, acting as a greenhouse gas. Carbon dioxide is soluble in water and is found in groundwater, lakes, ice caps, and seawater. When carbon dioxide dissolves in water, it forms carbonate and mainly bicarbonate (HCO−
3), which causes ocean acidification as atmospheric CO2 levels increase.Carbon Dioxide Fire Extinguisher

It is a trace gas in Earth’s atmosphere at 421 parts per million (ppm), or about 0.04% by volume (as of May 2022), having risen from pre-industrial levels of 280 ppm.[10][11] Burning fossil fuels is the primary cause of these increased CO2 concentrations and also the primary cause of climate change.[12]

Its concentration in Earth’s pre-industrial atmosphere since late in the Precambrian was regulated by organisms and geological phenomena. Plants, algae and cyanobacteria use energy from sunlight to synthesize carbohydrates from carbon dioxide and water in a process called photosynthesis, which produces oxygen as a waste product.[13] In turn, oxygen is consumed and CO2 is released as waste by all aerobic organisms when they metabolize organic compounds to produce energy by respiration.[14] CO2 is released from organic materials when they decay or combust, such as in forest fires. Since plants require CO2 for photosynthesis, and humans and animals depend on plants for food, CO2 is necessary for the survival of life on earth.Carbon Dioxide Fire Extinguisher

Carbon dioxide is 53% more dense than dry air, but is long lived and thoroughly mixes in the atmosphere. About half of excess CO2 emissions to the atmosphere are absorbed by land and ocean carbon sinks.[15] These sinks can become saturated and are volatile, as decay and wildfires result in the CO2 being released back into the atmosphere.[16] CO2 is eventually sequestered (stored for the long term) in rocks and organic deposits like coal, petroleum and natural gas. Sequestered CO2 is released into the atmosphere through burning fossil fuels or naturally by volcanoes, hot springs, geysers, and when carbonate rocks dissolve in water or react with acids.

CO2 is a versatile industrial material, used, for example, as an inert gas in welding and fire extinguishers, as a pressurizing gas in air guns and oil recovery, and as a supercritical fluid solvent in decaffeination of coffee and supercritical drying.[17] It is a byproduct of fermentation of sugars in bread, beer and wine making, and is added to carbonated beverages like seltzer and beer for effervescence. It has a sharp and acidic odor and generates the taste of soda water in the mouth,[18] but at normally encountered concentrations it is odorless
he symmetry of a carbon dioxide molecule is linear and unsymmetrical at its equilibrium geometry. The length of the carbon-oxygen bond in carbon dioxide is 116.3 pm, noticeably shorter than the roughly 140-pm length of a typical single C–O bond, and shorter than most other C–O multiply bonded functional groups such as carbons.[19] Since it is unsymmetrical, the molecule has no electric dipole moment.
Stretching and bending oscillations of the CO2 carbon dioxide molecule. Upper left: symmetric stretching. Upper right: anti symmetric stretching. Lower line: degenerate pair of bending modes.

As a linear diatomic molecule, CO2 has four vibrational modes as shown in the diagram. In the symmetric and the anti symmetric stretching modes, the atoms move along the axis of the molecule. There are two bending modes, which are degenerate, meaning that they have the same frequency and same energy, because of the symmetry of the molecule. When a molecule touches a surface or touches another molecule, the two bending modes can differ in frequency because the interaction is different for the two modes. Some of the vibrational modes are observed in the infrared (IR) spectrum: the antisymmetric stretching mode at wave number 2349 cm−1 (wavelength 4.25 μm) and the degenerate pair of bending modes at 667 cm−1 (wavelength 15 μm). The symmetric stretching mode does not create an electric dipole so is not observed in IR spectroscopy, but it is detected in by Raman spectroscopy at 1388 cm−1 Carbon Dioxide Fire Extinguisher

In the gas phase, carbon dioxide molecules undergo significant vibrational motions and do not keep a fixed structure. However, in a Coulomb explosion imaging experiment, an instantaneous image of the molecular structure can be deduced. Such an experiment[21] has been performed for carbon dioxide. The result of this experiment, and the conclusion of theoretical calculations[22] based on an ab initio potential energy surface of the molecule, is that none of the molecules in the gas phase are ever exactly linear. This counter-intuitive result is trivially due to the fact that the nuclear motion volume element vanishes for linear geometries.[22] This is so for all molecules
Carbon dioxide is colorless. At low concentrations, the gas is odorless; however, at sufficiently high concentrations, it has a sharp, acidic odor.[1] At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m3, about 1.53 times that of air.[27]

Carbon dioxide has no liquid state at pressures below 0.51795(10) MPa2 atm). At a pressure of 1 atm (0.101325 MPa), the gas deposits directly to a solid at temperatures below 194.6855(30) K2 °C) and the solid sublimes directly to a gas above this temperature. In its solid state, carbon dioxide is commonly called dry ice.
Pressure–temperature phase diagram of carbon dioxide. Note that it is a log-lin chart.

Liquid carbon dioxide forms only at pressures above 0.51795(10) MPa2 atm); the triple point of carbon dioxide is 216.592(3) K2 °C) at 0.51795(10) MPa2 atm) (see phase diagram). The critical point is 304.128(15) K2 °C) at 7.3773(30) MPa2 atm). Another form of solid carbon dioxide observed at high pressure is an amorphous glass-like solid.[28] This form of glass, called carbonia, is produced by supercooling heated CO2 at extreme pressures (40–48 GPa, or about 400,000 atmospheres) in a diamond anvil. This discovery confirmed the theory that carbon dioxide could exist in a glass state similar to other members of its elemental family, like silicon dioxide (silica glass) and germanium dioxide. Unlike silica and germania glasses, however, carbonia glass is not stable at normal pressures and reverts to gas when pressure is released.

At temperatures and pressures above the critical point, carbon dioxide behaves as a supercritical fluid known as supercritical carbon dioxide.

Table of thermal and physical properties of saturated liquid carbon dioxide
RuBisCO is thought to be the single most abundant protein on Earth.[31]

Phototrophs use the products of their photosynthesis as internal food sources and as raw material for the biosynthesis of more complex organic molecules, such as polysaccharides, nucleic acids, and proteins. These are used for their own growth, and also as the basis of the food chains and webs that feed other organisms, including animals such as ourselves. Some important phototrophs, the coccolithophores synthesise hard calcium carbonate scales.[32] A globally significant species of coccolithophore is Emiliania huxleyi whose calcite scales have formed the basis of many sedimentary rocks such as limestone, where what was previously atmospheric carbon can remain fixed for geological timescales.
Overview of photosynthesis and respiration. Carbon dioxide (at right), together with water, form oxygen and organic compounds (at left) by photosynthesis, which can be respired to water and (CO2).

Plants can grow as much as 50% faster in concentrations of 1,000 ppm CO2 when compared with ambient conditions, though this assumes no change in climate and no limitation on other nutrients.[33] Elevated CO2 levels cause increased growth reflected in the harvestable yield of crops, with wheat, rice and soybean all showing increases in yield of 12–14% under elevated CO2 in FACE experiments.[34][35]

Increased atmospheric CO2 concentrations result in fewer stomata developing on plants[36] which leads to reduced water usage and increased water-use efficiency.[37] Studies using FACE have shown that CO2 enrichment leads to decreased concentrations of micronutrients in crop plants.[38] This may have knock-on effects on other parts of ecosystems as herbivores will need to eat more food to gain the same amount of protein.[39]

The concentration of secondary metabolites such as phenomenological and flavonoids can also be altered in plants exposed to high concentrations of CO2.[40][41]

Plants also emit CO2 during respiration, and so the majority of plants and algae, which use C3 photosynthesis, are only net absorbers during the day. Though a growing forest will absorb many tons of CO2 each year, a mature forest will produce as much CO2 from respiration and decomposition of dead specimens (e.g., fallen branches) as is used in photosynthesis in growing plants.[42] Contrary to the long-standing view that they are carbon neutral, mature forests can continue to accumulate carbon[43] and remain valuable carbon sinks, helping to maintain the carbon balance of Earth’s atmosphere. Additionally, and crucially to life on earth, photosynthesis by phytoplankton consumes dissolved CO2 in the upper ocean and thereby promotes the absorption of CO2 from the atmosphere

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