Carbon dioxide

Carbon dioxide



IUPAC name Carbon dioxide
Systematic name Dioxidocarbon
Other names Carbonic acid gas Carbonic anhydride Carbonic oxide Carbon oxide Carbon(IV) oxide Dry ice (solid phase)
Identifiers
CAS number	124-38-9^Y
PubChem	280
ChemSpider	274^Y
UNII	142M471B3J^Y
EC number	204-696-9
UN number	1013
KEGG	D00004^Y
MeSH	Carbon+dioxide
ChEBI	CHEBI:16526^Y
RTECS number	FF6400000
ATC code	V03AN02
Beilstein Reference	1900390
Gmelin Reference	989
3DMet	B01131
Jmol-3D images	Image 1 Image 2
SMILES O=C=O C(=O)=O
InChI InChI=1S/CO2/c2-1-3^Y Key: CURLTUGMZLYLDI-UHFFFAOYSA-N^Y InChI=1/CO2/c2-1-3 Key: CURLTUGMZLYLDI-UHFFFAOYAO
Properties
Molecular formula	CO₂
Molar mass	44.01 g mol⁻¹
Exact mass	43.989829244 g mol⁻¹
Appearance	Colorless gas
Odor	Odorless
Density	1.562 g/mL (solid at 1 atm and −78.5 °C) 0.770 g/mL (liquid at 56 atm and 20 °C) 1.977 g/L (gas at 1 atm and 0 °C)
Melting point	-78 °C, 194.7 K, -109 °F (subl.)
Boiling point	-57 °C, 216.6 K, -70 °F (at 5.185 bar)
Solubility in water	1.45 g/L at 25 °C, 100 kPa
Acidity (pK_a)	6.35, 10.33
Refractive index (n_D)	1.1120
Viscosity	0.07 cP at −78 °C
Dipole moment	zero
Structure
Molecular shape	linear
Hazards
MSDS	External MSDS
NFPA 704	0 2 0
Related compounds
Other anions	Carbon disulfide Carbon diselenide
Other cations	Silicon dioxide Germanium dioxide Tin dioxide Lead dioxide
Related carbon oxides	Carbon monoxide Carbon suboxide Dicarbon monoxide Carbon trioxide
Related compounds	Carbonic acid Carbonyl sulfide
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Y dioxide (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Carbon dioxide (chemical formula CO₂) is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state, as a trace gas at a concentration of 0.039% by volume.

As part of the carbon cycle known as photosynthesis, plants, algae, and cyanobacteria absorb carbon dioxide, light, and water to produce carbohydrate energy for themselves and oxygen as a waste product.^[1] But in darkness photosynthesis cannot occur, and during the resultant respiration small amounts of carbon dioxide are produced.^[2] Carbon dioxide also is a by-product of combustion; is emitted from volcanoes, hot springs, and geysers; and is freed from carbonate rocks by dissolution.

As of July 2011^[update], carbon dioxide in the Earth's atmosphere is at a concentration of 392 ppm by volume.^[3] Atmospheric concentrations of carbon dioxide fluctuate slightly with the change of the seasons, driven primarily by seasonal plant growth in the Northern Hemisphere. Concentrations of carbon dioxide fall during the northern spring and summer as plants consume the gas, and rise during the northern autumn and winter as plants go dormant, die and decay. Taking all this into account, the concentration of CO₂ grew by about 2 ppm in 2009.^[4] Carbon dioxide is a greenhouse gas as it transmits visible light but absorbs strongly in the infrared and near-infrared, before slowly re-emitting the infrared at the same wavelength as what was absorbed.

Before the advent of human-caused release of carbon dioxide to the atmosphere, concentrations tended to increase with increasing global temperatures, acting as a positive feedback for changes induced by other processes such as orbital cycles.^[5] There is a seasonal cycle in CO₂ concentration associated primarily with the Northern Hemisphere growing season.^[6]

Carbon dioxide has no liquid state at pressures below 5.1 standard atmospheres (520 kPa). At 1 atmosphere (near mean sea level pressure), the gas deposits directly to a solid at temperatures below −78 °C (−108 °F; 195 K) and the solid sublimes directly to a gas above −78 °C. In its solid state, carbon dioxide is commonly called dry ice.

CO₂ is an acidic oxide: an aqueous solution turns litmus from blue to pink. It is the anhydride of carbonic acid, an acid which is unstable in aqueous solution, from which it cannot be concentrated. In organisms carbonic acid production is catalysed by the enzyme, carbonic anhydrase.

CO₂ + H₂O

H₂CO₃

CO₂ is toxic in higher concentrations: 1% (10,000 ppm) will make some people feel drowsy.^[7] Concentrations of 7% to 10% cause dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour.^[8]

Chemical and physical properties

Carbon dioxide pressure-temperature phase diagram showing the triple point and critical point of carbon dioxide

For more details on this topic, see Carbon dioxide (data page).

Carbon dioxide is colorless. At low concentrations, the gas is odorless. At higher concentrations it has a sharp, acidic odor. It can cause asphyxiation and irritation. When inhaled at concentrations much higher than usual atmospheric levels, it can produce a sour taste in the mouth and a stinging sensation in the nose and throat. These effects result from the gas dissolving in the mucous membranes and saliva, forming a weak solution of carbonic acid. This sensation can also occur during an attempt to stifle a burp after drinking a carbonated beverage. Amounts above 5,000 ppm are considered very unhealthy, and those above about 50,000 ppm (equal to 5% by volume) are considered dangerous to animal life.^[9]

At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m³, 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, and as it is fully oxidized, it is moderately reactive and is non-flammable, but will support the combustion of metals such as magnesium.

Above −78.51 °C (−109.32 °F), carbon dioxide changes directly from a solid phase to a gaseous phase through sublimation, or from gaseous to solid through deposition. Solid carbon dioxide is commonly called "dry ice", a generic trademark. It was first observed in 1825 by the French chemist Charles Thilorier. Dry ice is commonly used as a cooling agent, and it is relatively inexpensive. A convenient property for this purpose is that solid carbon dioxide sublimes directly into the gas phase, leaving no liquid. It can often be found in grocery stores and laboratories and is also used in the shipping industry. The largest non-cooling use for dry ice is blast cleaning.

Liquid carbon dioxide forms only at pressures above 5.1 atm; the triple point of carbon dioxide is about 518 kPa at −56.6 °C (see phase diagram, above). The critical point is 7.38 MPa at 31.1 °C.^[10]

Another form of solid carbon dioxide, an amorphous glass-like solid, is known, although not at atmospheric pressure.^[11] This form of glass, called carbonia, was produced by supercooling heated CO₂ at extreme pressure (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 (silica glass) and germanium. Unlike silica and germania glasses, however, carbonia glass is not stable at normal pressures and reverts back to gas when pressure is released.

History

Crystal structure of dry ice

Carbon dioxide was one of the first gases to be described as a substance distinct from air. In the seventeenth century, the Flemish chemist Jan Baptist van Helmont observed that when he burned charcoal in a closed vessel, the mass of the resulting ash was much less than that of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" or "wild spirit" (spiritus sylvestre).^{[citation needed]}

The properties of carbon dioxide were studied more thoroughly in the 1750s by the Scottish physician Joseph Black. He found that limestone (calcium carbonate) could be heated or treated with acids to yield a gas he called "fixed air." He observed that the fixed air was denser than air and supported neither flame nor animal life. Black also found that when bubbled through an aqueous solution of lime (calcium hydroxide), it would precipitate calcium carbonate. He used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, English chemist Joseph Priestley published a paper entitled Impregnating Water with Fixed Air in which he described a process of dripping sulfuric acid (or oil of vitriol as Priestley knew it) on chalk in order to produce carbon dioxide, and forcing the gas to dissolve by agitating a bowl of water in contact with the gas.^[12] This was the invention of Soda water.

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and Michael Faraday.^[13] The earliest description of solid carbon dioxide was given by Charles Thilorier, who in 1834 opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid CO₂.^[14]

Isolation and production

Carbon dioxide can be obtained by or from air distillation, however, this method is inefficient. A variety of chemical routes to carbon dioxide are known, such as the reaction between most acids and most metal carbonates. For example, the reaction between hydrochloric acid and calcium carbonate (limestone or chalk) is depicted below:

2 HCl + CaCO₃ → CaCl₂ + H₂CO₃

The carbonic acid (H₂CO₃) then decomposes to water and CO₂. Such reactions are accompanied by foaming or bubbling, or both. In industry such reactions are widespread because they can be used to neutralize waste acid streams.

The production of quicklime (CaO), a chemical that has widespread use, from limestone by heating at about 850 °C also produces CO₂:

CaCO₃ → CaO + CO₂

The combustion of all carbon-containing fuels, such as methane (natural gas), petroleum distillates (gasoline, diesel, kerosene, propane), but also of coal and wood, will yield carbon dioxide and, in most cases, water. As an example the chemical reaction between methane and oxygen is given below.

CH₄ + 2 O₂ → CO₂ + 2 H₂O

Iron is reduced from its oxides with coke in a blast furnace, producing pig iron and carbon dioxide:^[15]

Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂

Yeast metabolizes sugar to produce carbon dioxide and ethanol, also known as alcohol, in the production of wines, beers and other spirits, but also in the production of bioethanol:

C₆H₁₂O₆ → 2 CO₂ + 2 C₂H₅OH

All aerobic organisms produce CO₂ when they oxidize carbohydrates, fatty acids, and proteins in the mitochondria of cells. The large number of reactions involved are exceedingly complex and not described easily. Refer to (cellular respiration, anaerobic respiration and photosynthesis). Photoautotrophs (i.e. plants, cyanobacteria) use another modus operandi: Plants absorb CO₂ from the air, and, together with water, react it to form carbohydrates:

nCO₂ + nH₂O → (CH₂O)_n + nO₂

Carbon dioxide is soluble in water, in which it spontaneously interconverts between CO₂ and H₂CO₃ (carbonic acid). The relative concentrations of CO₂, H₂CO₃, and the deprotonated forms HCO−
3 (bicarbonate) and CO2−
3(carbonate) depend on the pH. In neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater, while in very alkaline water (pH > 10.4) the predominant (>50%) form is carbonate. The bicarbonate and carbonate forms are very soluble, such that air-equilibrated ocean water (mildly alkaline with typical pH = 8.2 – 8.5) contains about 120 mg of bicarbonate per liter.

Industrial production

Industrial carbon dioxide is produced mainly from six processes:^[16]

Directly from natural carbon dioxide springs, where it is produced by the action of acidified water on limestone or dolomite.
As a by-product of hydrogen production plants, where methane is converted to CO₂;
From combustion of fossil fuels and wood;
As a by-product of fermentation of sugar in the brewing of beer, whisky and other alcoholic beverages;
From thermal decomposition of limestone, CaCO₃, in the manufacture of lime (Calcium oxide, CaO);

Uses

Carbon dioxide bubbles in a soft drink.

Carbon dioxide is used by the food industry, the oil industry, and the chemical industry.^[16] It is used in many consumer products that require pressurized gas because it is inexpensive and nonflammable, and because it undergoes a phase transition from gas to liquid at room temperature at an attainable pressure of approximately 60 bar (870 psi, 59 atm), allowing far more carbon dioxide to fit in a given container than otherwise would. Life jackets often contain canisters of pressured carbon dioxide for quick inflation. Aluminum capsules of CO₂ are also sold as supplies of compressed gas for airguns, paintball markers, inflating bicycle tires, and for making carbonated water. Rapid vaporization of liquid carbon dioxide is used for blasting in coal mines. High concentrations of carbon dioxide can also be used to kill pests. Liquid carbon dioxide is used in supercritical drying of some food products and technological materials, in the preparation of specimens for scanning electron microscopy and in the decaffeination of coffee beans.

Foods

Carbon dioxide is a food additive used as a propellant and acidity regulator in the food industry. It is approved for usage in the EU^[17] (listed as E number E290), USA^[18] and Australia and New Zealand^[19] (listed by its INS number 290).

A candy called Pop Rocks is pressurized with carbon dioxide gas at about 40 bar (580 psi). When placed in the mouth, it dissolves (just like other hard candy) and releases the gas bubbles with an audible pop.

Leavening agents produce carbon dioxide to cause dough to rise. Baker's yeast produces carbon dioxide by fermentation of sugars within the dough, while chemical leaveners such as baking powder and baking soda release carbon dioxide when heated or if exposed to acids.

Beverages

Carbon dioxide is used to produce carbonated soft drinks and soda water. Traditionally, the carbonation in beer and sparkling wine came about through natural fermentation, but many manufacturers carbonate these drinks with carbon dioxide recovered from the fermentation process. In the case of bottled and kegged beer, recycled carbon dioxide carbonation is the most common method used. With the exception of British Real Ale, draught beer is usually transferred from kegs in a cold room or cellar to dispensing taps on the bar using pressurized carbon dioxide, sometimes mixed with nitrogen.

Wine making

Carbon dioxide in the form of dry ice is often used in the wine making process to cool down bunches of grapes quickly after picking to help prevent spontaneous fermentation by wild yeasts. The main advantage of using dry ice over regular water ice is that it cools the grapes without adding any additional water that may decrease the sugar concentration in the grape must, and therefore also decrease the alcohol concentration in the finished wine.

Dry ice is also used during the cold soak phase of the wine making process to keep grapes cool. The carbon dioxide gas that results from the sublimation of the dry ice tends to settle to the bottom of tanks because it is heavier than air. The settled carbon dioxide gas creates a hypoxic environment which helps to prevent bacteria from growing on the grapes until it is time to start the fermentation with the desired strain of yeast.

Carbon dioxide is also used to create a hypoxic environment for carbonic maceration, the process used to produce Beaujolais wine.

Carbon dioxide is sometimes used to top up wine bottles or other storage vessels such as barrels to prevent oxidation, though it has the problem that it can dissolve into the wine, making a previously still wine slightly fizzy. For this reason, other gases such as nitrogen or argon are preferred for this process by professional wine makers.

Pneumatic systems

Carbon dioxide is one of the most commonly used compressed gases for pneumatic (pressurized gas) systems in portable pressure tools and combat robots.

Fire extinguisher

Carbon dioxide extinguishes flames, and some fire extinguishers, especially those designed for electrical fires, contain liquid carbon dioxide under pressure. Carbon dioxide extinguishers work well on small flammable liquid and electrical fires, but not on ordinary combustible fires, as it is so dry. Carbon dioxide has also been widely used as an extinguishing agent in fixed fire protection systems for local application of specific hazards and total flooding of a protected space.^[20]International Maritime Organization standards also recognize carbon dioxide systems for fire protection of ship holds and engine rooms. Carbon dioxide based fire protection systems have been linked to several deaths, because it does not support life in the concentrations used to extinguish fire (40% or so), however, it is not considered to be toxic to humans. A review of CO₂ systems identified 51 incidents between 1975 and the date of the report, causing 72 deaths and 145 injuries.^[21]

Welding

Carbon dioxide also finds use as an atmosphere for welding, although in the welding arc, it reacts to oxidize most metals. Use in the automotive industry is common despite significant evidence that welds made in carbon dioxide are more brittle than those made in more inert atmospheres, and that such weld joints deteriorate over time because of the formation of carbonic acid. It is used as a welding gas primarily because it is much less expensive than more inert gases such as argon or helium.

When used for MIG welding, CO₂ use is sometimes referred to as MAG welding, for Metal Active Gas, as CO₂ can react at these high temperatures. It tends to produce a hotter puddle than truly inert atmospheres, improving the flow characteristics. Although, this may be due to atmospheric reactions occurring at the puddle site. This is usually the opposite of the desired effect when welding, as it tends to embrittle the site, but may not be a problem for general mild steel welding, where ultimate ductility is not a major concern.

Pharmaceutical and other chemical processing

Liquid carbon dioxide is a good solvent for many lipophilic organic compounds and is used to remove caffeine from coffee. Carbon dioxide has attracted attention in the pharmaceutical and other chemical processing industries as a less toxic alternative to more traditional solvents such as organochlorides. It is used by some dry cleaners for this reason. (See green chemistry.)

Carbon dioxide is used as an ingredient in the production of urea, carbonates and bicarbonates, and sodium salicylate.^[22] Carbon dioxide is known to react with Grignard reagents to form carboxylic acids. In a metal carbon dioxide complexes, CO₂ serves as a ligand, which can facilitate the conversion of CO₂ to other chemicals.

Agricultural and biological applications

Plants require carbon dioxide to conduct photosynthesis. Greenhouses may (if of large size, must) enrich their atmospheres with photosynthesis-related drop (by a factor less than two) in carbon dioxide concentration in a greenhouse compartment would kill green plants, or, at least, completely stop their growth. At very high concentrations (a factor of 100 or more higher than its atmospheric concentration), carbon dioxide can be toxic to animal life, so raising the concentration to 10,000 ppm (1%) or higher for several hours will eliminate pests such as whiteflies and spider mites in a greenhouse.^[25] Carbon dioxide is used in greenhouses as the main carbon source for Spirulina algae.

In medicine, up to 5% carbon dioxide (130 times the atmospheric concentration) is added to oxygen for stimulation of breathing after apnea and to stabilize the O₂/CO₂ balance in blood.

It has been proposed that carbon dioxide from power generation be bubbled into ponds to grow algae that could then be converted into biodiesel fuel.^[26]

Lasers

A carbon dioxide laser.

A common type of industrial gas laser is the carbon dioxide laser, which is one of the earliest type of lasers.

Oil recovery

Carbon dioxide is used in enhanced oil recovery where it is injected into or adjacent to producing oil wells, usually under supercritical conditions. It acts as both a pressurizing agent and, when dissolved into the underground crude oil, significantly reduces its viscosity, enabling the oil to flow more rapidly through the earth to the removal well.^[27] In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points.

Refrigerant

Liquid and solid carbon dioxide are important refrigerants, especially in the food industry, where they are employed during the transportation and storage of ice cream and other frozen foods. Solid carbon dioxide is called "dry ice" and is used for small shipments where refrigeration equipment is not practical. Solid carbon dioxide is always below −78 °C at regular atmospheric pressure, regardless of the air temperature.

Liquid carbon dioxide (industry nomenclature R744 or R-744) was used as a refrigerant prior to the discovery of R-12 and may enjoy a renaissance due to the fact that r134a contributes to climate change. Its physical properties are highly favorable for cooling, refrigeration, and heating purposes, having a high volumetric cooling capacity. Due to its operation at pressures of up to 130 bar (1880 psi), CO₂ systems require highly resistant components that have already been developed for mass production in many sectors. In automobile air conditioning, in more than 90% of all driving conditions for latitudes higher than 50°, R744 operates more efficiently than systems using R-134a. Its environmental advantages (GWP of 1, non-ozone depleting, non-toxic, non-flammable) could make it the future working fluid to replace current HFCs in cars, supermarkets, hot water heat pumps, among others. Coca-Cola has fielded CO₂-based beverage coolers and the U.S. Army is interested in CO₂ refrigeration and heating technology.^[28]^[29]

The global automobile industry is expected to decide on the next-generation refrigerant in car air conditioning. CO₂ is one discussed option.(see The Cool War)

Coal bed methane recovery

In enhanced coal bed methane recovery, carbon dioxide is pumped into the coal seam to displace methane.^[30]

pH control

Carbon dioxide can be used as a mean of controlling the pH of swimming pools, by continuously adding gas to the water, thus keeping the pH level from rising. Among the advantages of this is the avoidance of handling (more hazardous) acids.

CO₂ is also used in the keeping of reef aquaria, where it is commonly used in calcium reactors to temporarily lower the pH of water being passed over calcium carbonate in order to allow the calcium carbonate to dissolve into the water more freely where it is used by some corals to build their skeleton.

In the Earth's atmosphere

The Keeling Curve of atmospheric CO₂ concentrations measured at Mauna Loa Observatory.

Main article: Carbon dioxide in Earth's atmosphere

Main article: Carbon cycle

Carbon dioxide in earth's atmosphere is considered a trace gas currently occurring at an average concentration of about 390 parts per million by volume or 591 parts per million by mass.^[31] The total mass of atmospheric carbon dioxide is 3.16×10¹⁵ kg (about 3,000 gigatonnes). Its concentration varies seasonally (see graph at right) and also considerably on a regional basis, especially near the ground. In urban areas concentrations are generally higher and indoors they can reach 10 times background levels. Carbon dioxide is a greenhouse gas.

Yearly increase of atmospheric CO₂: In the 1960s, the average annual increase was 37% of the 2000–2007 average.^[32]

Five hundred million years ago carbon dioxide was 20 times more prevalent than today, decreasing to 4–5 times during the Jurassic period and then slowly declining with a particularly swift reduction occurring 49 million years ago.^[33]^[34] Human activities such as the combustion of fossil fuels and deforestation have caused the atmospheric concentration of carbon dioxide to increase by about 35% since the beginning of the age of industrialization.^[35]

Up to 40% of the gas emitted by some volcanoes during subaerial eruptions is carbon dioxide.^[36] It is estimated that volcanoes release about 130–230 million tonnes (145–255 million tons) of CO₂ into the atmosphere each year. Carbon dioxide is also produced by hot springs such as those at the Bossoleto site near Rapolano Terme in Tuscany, Italy. Here, in a bowl-shaped depression of about 100 m diameter, local concentrations of CO₂ rise to above 75% overnight, sufficient to kill insects and small animals, but it warms rapidly when sunlit and the gas is dispersed by convection during the day.^[37] Locally high concentrations of CO₂, produced by disturbance of deep lake water saturated with CO₂ are thought to have caused 37 fatalities at Lake Monoun, Cameroon in 1984 and 1700 casualties at Lake Nyos, Cameroon in 1986.^[38] Emissions of CO₂ by human activities are currently more than 130 times greater than the quantity emitted by volcanoes, amounting to about 27 billion tonnes per year.^[39]

In the oceans

Main article: Carbon cycle

There is about fifty times as much carbon dissolved in the sea water of the oceans in the form of CO₂ and carbonic acid, bicarbonate and carbonate ions as exists in the atmosphere. The oceans act as an enormous carbon sink, and have taken up about a third of CO₂ emitted by human activity.^[40] Gas solubility decreases as the temperature of water increases (except when both pressure exceeds 300 bar and temperature exceeds 393 K, only found near deep geothermal vents)^[41] and therefore the rate of uptake from the atmosphere decreases as ocean temperatures rise.

Most of the CO₂ taken up by the ocean, which is about 30% of the total released into the atmosphere,^[42] forms carbonic acid in equilibrium with bicarbonate and carbonate ions. Some is consumed in photosynthesis by organisms in the water, and a small proportion of that sinks and leaves the carbon cycle. Increased CO₂ in the atmosphere has led to decreasing alkalinity of seawater and there is concern that this may adversely affect organisms living in the water. In particular, with decreasing alkalinity, the availability of carbonates for forming shells decreases,^[43] although there's evidence of increased shell production by certain species under increased CO₂ content.^[44]

NOAA states in their May 2008 "State of the science fact sheet for ocean acidification" that:
"The oceans have absorbed about 50% of the carbon dioxide (CO₂) released from the burning of fossil fuels, resulting in chemical reactions that lower ocean pH. This has caused an increase in hydrogen ion (acidity) of about 30% since the start of the industrial age through a process known as “ocean acidification.” A growing number of studies have demonstrated adverse impacts on marine organisms, including:

The rate at which reef-building corals produce their skeletons decreases, while production of numerous varieties of jellyfish increases.
The ability of marine algae and free-swimming zooplankton to maintain protective shells is reduced.
The survival of larval marine species, including commercial fish and shellfish, is reduced."

Also, the Intergovernmental Panel on Climate Change (IPCC) writes in their Climate Change 2007: Synthesis Report:^[45]
"The uptake of anthropogenic carbon since 1750 has led to the ocean becoming more acidic with an average decrease in pH of 0.1 units. Increasing atmospheric CO₂ concentrations lead [sic] to further acidification [...] While the effects of observed ocean acidification on the marine biosphere are as yet undocumented, the progressive acidification of oceans is expected to have negative impacts on marine shell-forming organisms (e.g. corals) and their dependent species."

Some marine calcifying organisms (including coral reefs) have been singled out by major research agencies, including NOAA, OSPAR commission, NANOOS and the IPCC, because their most current research shows that ocean acidification should be expected to impact them negatively.^[46]

The Champagne hydrothermal vent, found at the Northwest Eifuku volcano at Marianas Trench Marine National Monument, produces almost pure liquid carbon dioxide, one of only two known sites in the world.^[47]

Biological role

Carbon dioxide is an end product in organisms that obtain energy from breaking down sugars, fats and amino acids 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 from the body's tissues to the lungs where it is exhaled. In plants using photosynthesis, carbon dioxide is absorbed from the atmosphere.

Photosynthesis and carbon fixation

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).

Carbon fixation is the removal of carbon dioxide from the air and its incorporation into solid compounds. Plants, algae, and many species of bacteria (cyanobacteria) fix carbon and create their own food by photosynthesis. Photosynthesis uses carbon dioxide and water to produce sugars and occasionally other organic compounds, releasing oxygen as a waste product. These phototrophs use the products of their photosynthesis as internal food sources and as raw material for the construction of more complex organic molecules, such as polysaccharides, nucleic acids and proteins. These are used for their own growth, and also as the basis for the food chains and webs whereby other organisms, including animals such as ourselves, are fed. 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 up to 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.^[48] Research has shown that 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.^[49]^[50]

Studies have shown that increased CO₂ leads to fewer stomata developing on plants^[51] which leads to reduced water usage.^[52] Studies using FACE have shown that increases in CO₂ lead to decreased concentration of micronutrients in crop plants.^[53] 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.^[54]

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, the World Bank writes that a mature forest will produce as much CO₂ from respiration and decomposition of dead specimens (e.g., fallen branches) as is used in biosynthesis in growing plants.^[55] However six experts in biochemistry, biogeology, forestry and related areas writing in the science journal Nature that "Our results demonstrate that old-growth forests can continue to accumulate carbon, contrary to the long-standing view that they are carbon neutral." ^[56] Mature forests are valuable carbon sinks, helping maintain balance in the 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.^[57]

Toxicity

Human physiology

References

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^ Food Factories. www.legacyproject.org. Retrieved on 2011-10-10.
^ Mauna Loa CO₂ annual mean data from NOAA. "Trend" data was used. See also: Trends in Carbon Dioxide from NOAA.
^ "Annual Mean Growth Rate for Mauna Loa, Hawaii". Trends in Atmospheric Carbon Dioxide. NOAA Earth System Research Laboratory. http://www.esrl.noaa.gov/gmd/ccgg/trends/#mlo_growth. Retrieved 28 April 2010.
^ Genthon, G.; Barnola, J. M.; Raynaud, D.; Lorius, C.; Jouzel, J.; Barkov, N. I.; Korotkevich, Y. S.; Kotlyakov, V. M. (1987). "Vostok ice core: climatic response to CO2 and orbital forcing changes over the last climatic cycle". Nature 329 (6138): 414. Bibcode 1987Natur.329..414G. doi:10.1038/329414a0. edit
^ Enting, I.G., 1987: Interannual variation in the seasonal cycle of carbon dioxide concentration at Mauna Loa. Journal of Geophysical Research 92:D5, 5497–5504.
^ ^a ^b ^c Toxicity of Carbon Dioxide Gas Exposure, CO2 Poisoning Symptoms, Carbon Dioxide Exposure Limits, and Links to Toxic Gas Testing Procedures By Daniel Friedman – InspectAPedia
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^ Staff (16 August 2006). "Carbon dioxide: IDLH Documentation". National Institute for Occupational Safety and Health. http://www.cdc.gov/niosh/idlh/124389.html. Retrieved 2007-07-05.
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External links

v · d · eOxocarbons

Common oxides	CO₂ · CO

Exotic oxides	CO₃ · CO₄ · C₂O · C₂O₂ · C₂O₃ · C₂O₄ · C₃O₂ · C₄O₂ · C₄O₄ · C₄O₆ · C₅O₂ · C₅O₅ · C₆O₆ · C₁₂O₆ · C₁₂O₉

Compounds derived from oxides	Metal carbonyls · Carbonic acid · Bicarbonates · Carbonates · Dicarbonates · Tricarbonates

Global warming and climate change

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Carbon-free energy	Efficient energy use Renewable energy Nuclear power Carbon capture and storage Low-carbon economy

Other	Geoengineering Carbon dioxide removal Carbon sink Individual and political action on climate change Climate change mitigation scenarios Reducing Emissions from Deforestation and Forest Degradation

Proposed adaptations

Strategies	Damming glacial lakes Drought tolerance Irrigation investment Rainwater storage Sustainable development Water desalination Weather control

Programmes	Avoiding Dangerous Climate Change Land Allocation Decision Support System

Categories:

Carbon dioxide
Oxocarbons
Acid anhydrides
Acidic oxides
Greenhouse gases
Propellants
Household chemicals
Inorganic solvents
Refrigerants
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Laser gain media

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carbon dioxide — UK US noun [U] (also carbon, ABBREVIATION CO2) ► ENVIRONMENT the gas formed when carbon is burned, or when people or animals breathe out. Carbon dioxide is a greenhouse gas: »The UK plans to reduce carbon dioxide emissions by 60% by 2050. »Hotter … Financial and business terms
carbon dioxide — n a heavy colorless gas CO2 that does not support combustion, dissolves in water to form carbonic acid, is formed esp. in animal respiration and in the decay or combustion of animal and vegetable matter, is absorbed from the air by plants in… … Medical dictionary
Carbon dioxide — Carbon Car bon (k[aum]r b[o^]n), n. [F. carbone, fr. L. carbo coal; cf. Skr. [,c]r[=a] to cook.] (Chem.) 1. An elementary substance, not metallic in its nature, which is present in all organic compounds. Atomic weight 11.97. Symbol C. it is… … The Collaborative International Dictionary of English
carbon dioxide — n. a colorless, odorless, incombustible gas, CO2, somewhat heavier than air, that is a product of respiration and combustion: produced commercially and used widely in fire extinguishers, carbonated beverages, etc.: in photosynthesis, carbon… … English World dictionary
carbon dioxide — , carbon monoxide Carbon dioxide is the gas people exhale; carbon monoxide is the highly poisonous gas associated with car exhausts … Bryson’s dictionary for writers and editors
Carbon dioxide — Dioxide Di*ox ide (?; 104), n. [Pref. di + oxide.] (Chem.) (a) An oxide containing two atoms of oxygen in each molecule; binoxide. (b) An oxide containing but one atom or equivalent of oxygen to two of a metal; a suboxide. [Obs.] [1913 Webster]… … The Collaborative International Dictionary of English
carbon dioxide — 1873, so called because it consists of one carbon and two oxygen atoms. Known since mid 18c. under the name fixed air … Etymology dictionary
carbon dioxide — ► NOUN ▪ a colourless, odourless gas produced by burning carbon and organic compounds and by respiration, and absorbed by plants in photosynthesis … English terms dictionary
carbon dioxide — a colorless, odorless, incombustible gas, CO2, present in the atmosphere and formed during respiration, usually obtained from coal, coke, or natural gas by combustion, from carbohydrates by fermentation, by reaction of acid with limestone or… … Universalium
carbon dioxide — noun a heavy odorless colorless gas formed during respiration and by the decomposition of organic substances; absorbed from the air by plants in photosynthesis (Freq. 1) • Syn: ↑CO2, ↑carbonic acid gas • Derivationally related forms: ↑carbonate • … Useful english dictionary

Academic Dictionaries and Encyclopedias

Carbon dioxide

Contents

Chemical and physical properties

History