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Carbon dioxide ( chemical formula ) 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. In the air, carbon dioxide is transparent to visible light but absorbs infrared radiation, acting as a greenhouse gas. 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. Burning fossil fuels is the primary cause of these increased CO₂ concentrations and also the primary cause of climate change.IPCC (2022
Summary for policy makers
i
Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA Carbon dioxide is soluble in water and is found in groundwater, lakes, ice caps, and seawater. When carbon dioxide dissolves in water, it forms carbonic acid (H₂CO₃), which causes ocean acidification as atmospheric CO₂ levels increase.

As the source of available carbon in the carbon cycle, atmospheric CO₂ is the primary carbon source for life on Earth. Its concentration in Earth's pre-industrial atmosphere since late in the Precambrian has been 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. 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. 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 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. These sinks can become saturated and are volatile, as decay and wildfires result in the CO₂ being released back into the atmosphere. 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. It is also a feedstock for the synthesis of fuels and chemicals. It is an unwanted byproduct in many large scale oxidation processes, for example, in the production of acrylic acid (over 5 million tons/year). The frozen solid form of CO₂, known as dry ice, is used as a refrigerant and as an abrasive in dry-ice blasting. 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, but at normally encountered concentrations it is odorless.

Chemical and physical properties

Structure, bonding and molecular vibrations

The symmetry of a carbon dioxide molecule is linear and centrosymmetric 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 carbonyls. Since it is centrosymmetric, the molecule has no electric dipole moment.

As a linear triatomic molecule, has four vibrational modes as shown in the diagram. In the symmetric and the antisymmetric 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 wavenumber 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 (wavelength 7.2 μm).

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 has been performed for carbon dioxide.
The result of this experiment, and the conclusion of theoretical calculations 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.

In aqueous solution

Carbon dioxide is soluble in water, in which it reversibly forms (carbonic acid), which is a weak acid since its ionization in water is incomplete.
:CO2 + H2O <=> H2CO3

The hydration equilibrium constant of carbonic acid is, at 25 °C:
:$K_\mathrm = \frac = 1.70 \times 10^$
Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as molecules, not affecting the pH.

The relative concentrations of , , and the deprotonated forms ( bicarbonate) and ( carbonate) depend on the pH. As shown in a Bjerrum plot, in neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater. In very alkaline water (pH > 10.4), the predominant (>50%) form is carbonate. The oceans, being mildly alkaline with typical pH = 8.2–8.5, contain about 120 mg of bicarbonate per liter.

Being diprotic, carbonic acid has two acid dissociation constants, the first one for the dissociation into the bicarbonate (also called hydrogen carbonate) ion ():

:H2CO3 <=> HCO3- + H+
:''K''_a1 = ; p''K''_a1 = 3.6 at 25 °C.
This is the ''true'' first acid dissociation constant, defined as
:$K_\mathrm = \frac$
where the denominator includes only covalently bound and does not include hydrated . The much smaller and often-quoted value near is an ''apparent'' value calculated on the (incorrect) assumption that all dissolved is present as carbonic acid, so that
:$K_\mathrm=\frac$
Since most of the dissolved remains as molecules, ''K''_a1(apparent) has a much larger denominator and a much smaller value than the true ''K''_a1.

The bicarbonate ion is an amphoteric species that can act as an acid or as a base, depending on pH of the solution. At high pH, it dissociates significantly into the carbonate ion ():
:HCO3- <=> CO3^2- + H+
:''K''_a2 = ; p''K''_a2 = 10.329

In organisms carbonic acid production is catalysed by the enzyme, carbonic anhydrase.

Chemical reactions of CO₂

is a potent electrophile having an electrophilic reactivity that is comparable to benzaldehyde or strong α,β-unsaturated carbonyl compounds. However, unlike electrophiles of similar reactivity, the reactions of nucleophiles with are thermodynamically less favored and are often found to be highly reversible. Only very strong nucleophiles, like the carbanions provided by Grignard reagents and organolithium compounds react with to give carboxylates:
:MR + CO2 -> RCO2M
:where M = Li or Mg Br and R = alkyl or aryl.

In metal carbon dioxide complexes, serves as a ligand, which can facilitate the conversion of to other chemicals.

The reduction of to CO is ordinarily a difficult and slow reaction:
:CO2 + 2 e- + 2H+ -> CO + H2O

Photoautotrophs (i.e. plants and cyanobacteria) use the energy contained in sunlight to photosynthesize simple sugars from absorbed from the air and water:
: \mathitCO2 + \mathitH2O -> (CH2O)_\mathit + \mathitO2

The redox potential for this reaction near pH 7 is about −0.53 V ''versus'' the standard hydrogen electrode. The nickel-containing enzyme carbon monoxide dehydrogenase catalyses this process.

Physical properties

Carbon dioxide is colorless. At low concentrations the gas is odorless; however, at sufficiently high concentrations, it has a sharp, acidic odor. At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m³, about 1.53 times that of air.

Carbon dioxide has no liquid state at pressures below (). At a pressure of 1 atm (), the gas deposits directly to a solid at temperatures below () and the solid sublimes directly to a gas above this temperature. In its solid state, carbon dioxide is commonly called dry ice.

Liquid carbon dioxide forms only at pressures above (); the triple point of carbon dioxide is () at () (see phase diagram). The critical point is () at (). Another form of solid carbon dioxide observed at high pressure is an amorphous glass-like solid. This form of glass, called '' carbonia'', is produced by supercooling heated 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:

Table of thermal and physical properties of carbon dioxide (CO2) at atmospheric pressure:

Biological role

Carbon dioxide is an end product of cellular respiration in organisms that obtain energy by breaking down sugars, fats and amino acids with oxygen as part of their metabolism. This includes all plants, algae and animals and aerobic fungi and bacteria. In vertebrates, the carbon dioxide travels in the blood from the body's tissues to the skin (e.g., amphibians) or the gills (e.g., fish), from where it dissolves in the water, or to the lungs from where it is exhaled. During active photosynthesis, plants can absorb more carbon dioxide from the atmosphere than they release in respiration.

Photosynthesis and carbon fixation

Carbon fixation is a biochemical process by which atmospheric carbon dioxide is incorporated by plants, algae and (cyanobacteria) into energy-rich organic molecules such as glucose, thus creating their own food by photosynthesis. Photosynthesis uses carbon dioxide and water to produce sugars from which other organic compounds can be constructed, and oxygen is produced as a by-product.

Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly abbreviated to RuBisCO, is the enzyme involved in the first major step of carbon fixation, the production of two molecules of 3-phosphoglycerate from CO₂ and ribulose bisphosphate, as shown in the diagram at left.

RuBisCO is thought to be the single most abundant protein on Earth.

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. 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.

Plants can grow as much as 50 percent 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. 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.

Increased atmospheric CO₂ concentrations result in fewer stomata developing on plants which leads to reduced water usage and increased water-use efficiency. Studies using FACE have shown that CO₂ enrichment leads to decreased concentrations of micronutrients in crop plants. 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.

The concentration of secondary metabolites such as phenylpropanoids and flavonoids
can also be altered in plants exposed to high concentrations of CO₂.

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. Contrary to the long-standing view that they are carbon neutral, mature forests can continue to accumulate carbon 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.

Toxicity

Carbon dioxide content in fresh air (averaged between sea-level and 10 kPa level, i.e., about altitude) varies between 0.036% (360 ppm) and 0.041% (412 ppm), depending on the location.

CO₂ is an asphyxiant gas and not classified as toxic or harmful in accordance with Globally Harmonized System of Classification and Labelling of Chemicals standards of United Nations Economic Commission for Europe by using the OECD Guidelines for the Testing of Chemicals. In concentrations up to 1% (10,000 ppm), it will make some people feel drowsy and give the lungs a stuffy feeling. Concentrations of 7% to 10% (70,000 to 100,000 ppm) may cause suffocation, even in the presence of sufficient oxygen, manifesting as dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour. The physiological effects of acute carbon dioxide exposure are grouped together under the term hypercapnia, a subset of asphyxiation.

Because it is heavier than air, in locations where the gas seeps from the ground (due to sub-surface volcanic or geothermal activity) in relatively high concentrations, without the dispersing effects of wind, it can collect in sheltered/pocketed locations below average ground level, causing animals located therein to be suffocated. Carrion feeders attracted to the carcasses are then also killed. Children have been killed in the same way near the city of Goma by CO₂ emissions from the nearby volcano Mount Nyiragongo. The Swahili term for this phenomenon is .

Adaptation to increased concentrations of CO₂ occurs in humans, including modified breathing and kidney bicarbonate production, in order to balance the effects of blood acidification ( acidosis). Several studies suggested that 2.0 percent inspired concentrations could be used for closed air spaces (e.g. a submarine) since the adaptation is physiological and reversible, as deterioration in performance or in normal physical activity does not happen at this level of exposure for five days. Yet, other studies show a decrease in cognitive function even at much lower levels. Also, with ongoing respiratory acidosis, adaptation or compensatory mechanisms will be unable to reverse such condition.

Below 1%

There are few studies of the health effects of long-term continuous CO₂ exposure on humans and animals at levels below 1%. Occupational CO₂ exposure limits have been set in the United States at 0.5% (5000 ppm) for an eight-hour period. At this CO₂ concentration, International Space Station crew experienced headaches, lethargy, mental slowness, emotional irritation, and sleep disruption. Studies in animals at 0.5% CO₂ have demonstrated kidney calcification and bone loss after eight weeks of exposure. A study of humans exposed in 2.5 hour sessions demonstrated significant negative effects on cognitive abilities at concentrations as low as 0.1% (1000ppm) CO₂ likely due to CO₂ induced increases in cerebral blood flow. Another study observed a decline in basic activity level and information usage at 1000 ppm, when compared to 500 ppm. However a review of the literature found that most studies on the phenomenon of carbon dioxide induced cognitive impairment to have a small effect on high-level decision making and most of the studies were confounded by inadequate study designs, environmental comfort, uncertainties in exposure doses and differing cognitive assessments used. Similarly a study on the effects of the concentration of CO₂ in motorcycle helmets has been criticized for having dubious methodology in not noting the self-reports of motorcycle riders and taking measurements using mannequins. Further when normal motorcycle conditions were achieved (such as highway or city speeds) or the visor was raised the concentration of CO₂ declined to safe levels (0.2%).

Ventilation

Poor ventilation is one of the main causes of excessive CO₂ concentrations in closed spaces. Carbon dioxide differential above outdoor concentrations at steady state conditions (when the occupancy and ventilation system operation are sufficiently long that CO₂ concentration has stabilized) are sometimes used to estimate ventilation rates per person. Higher CO₂ concentrations are associated with occupant health, comfort and performance degradation. ASHRAE Standard 62.1–2007 ventilation rates may result in indoor concentrations up to 2,100 ppm above ambient outdoor conditions. Thus if the outdoor concentration is 400 ppm, indoor concentrations may reach 2,500 ppm with ventilation rates that meet this industry consensus standard. Concentrations in poorly ventilated spaces can be found even higher than this (range of 3,000 or 4,000 ppm).

Miners, who are particularly vulnerable to gas exposure due to insufficient ventilation, referred to mixtures of carbon dioxide and nitrogen as " blackdamp," "choke damp" or "stythe." Before more effective technologies were developed, miners would frequently monitor for dangerous levels of blackdamp and other gases in mine shafts by bringing a caged canary with them as they worked. The canary is more sensitive to asphyxiant gases than humans, and as it became unconscious would stop singing and fall off its perch. The Davy lamp

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