TheInfoList

Carbon dioxide (
chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas and ...
) is a colorless
gas Gas is one of the four fundamental states of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

with a density about 53% higher than that of dry air. Carbon dioxide
molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion I ...

s consist of a
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atom ...

covalently A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they s ...
double bond In chemistry, a double bond is a between two s involving four s as opposed to two in a . Double bonds occur most commonly between two carbon atoms, for example in s. Many double bonds exist between two different elements: for example, in a group ...

ed to two
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

atoms. It occurs naturally in
Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmosphere (0.04402961% at April 2019 concentration ). Number ...
as a
trace gas#REDIRECT Trace gas Trace gases are those gases in the atmosphere other than nitrogen Nitrogen is the chemical element with the Symbol (chemistry), symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel ...
. The current concentration is about 0.04% (412  ppm) by volume, having risen from pre-industrial levels of 280 ppm. Natural sources include
volcano A volcano is a rupture in the of a , such as , that allows hot , , and to escape from a below the surface. On Earth, volcanoes are most often found where are or , and most are found underwater. For example, a , such as the , has volcanoe ...

es,
hot spring A hot spring, hydrothermal spring, or geothermal spring is a spring Spring(s) may refer to: Common uses * Spring (season), a season of the year * Spring (device), a mechanical device that stores energy * Spring (hydrology), a natural source of w ...
s and
geyser A geyser (, ) is a spring Spring(s) may refer to: Common uses * Spring (season), a season of the year * Spring (device), a mechanical device that stores energy * Spring (hydrology), a natural source of water * Spring (mathematics), a geometric ...

s, and it is freed from
carbonate rock 250px, Carbonate ooids on the surface of a limestone; Carmel Formation (Middle Jurassic) of southern Utah, USA. Largest is 1.0 mm in diameter. Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major ...
s by
dissolution Dissolution may refer to: Arts and entertainment Books * Dissolution (Forgotten Realms novel), ''Dissolution'' (''Forgotten Realms'' novel), a 2002 fantasy novel by Richard Lee Byers * Dissolution (Sansom novel), ''Dissolution'' (Sansom novel), a 2 ...
in water and acids. Because carbon dioxide is soluble in water, it occurs naturally in
groundwater Groundwater is the water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

,
river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...

s and
lake A lake is an area filled with water, localized in a Depression (geology), basin, surrounded by land, and set apart from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the World Ocean, oc ...

s,
ice cap upright=1.35, Vatnajökull, Iceland ">Iceland.html" ;"title="Vatnajökull, Iceland">Vatnajökull, Iceland In glaciology, an ice cap is a mass of ice that covers less than of land area (usually covering a highland area). Larger ice masses coveri ...
s,
glacier A glacier (; ) is a persistent body of dense ice Ice is into a state. Depending on the presence of such as particles of soil or bubbles of air, it can appear transparent or a more or less bluish-white color. In the , ice is abunda ...

s and
seawater Seawater, or salt water, is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of E ...

. It is present in deposits of
petroleum Petroleum, also known as crude oil and oil, is a naturally occurring, yellowish-black liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isoc ...

and
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon In , a hydrocarbon is an consisting entirely of and . Hydrocarbons are examples of s. Hydrocarbons are generally colourless and hydrophobic wit ...

. Carbon dioxide has a sharp and acidic odor and generates the taste of
soda water Carbonated water (also known as soda water, sparkling water, fizzy water, water with gas or (especially in the United States, U.S.) as seltzer or seltzer water) is water containing dissolved carbon dioxide gas, either artificially injected und ...
in the mouth. However, at normally encountered concentrations it is odorless. As the source of available carbon in the
carbon cycle The carbon cycle is the biogeochemical cycle In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physi ...

,
atmospheric carbon dioxide Carbon dioxide Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% higher than that of dry air. Carbon dioxide molecules consist of a carbon atom covalent bond, covalently double bonded to two oxygen atoms. It occu ...
is the primary carbon source for and its concentration in Earth's pre-industrial atmosphere since late in the
Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale The geologi ...

has been regulated by
photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

organisms and geological phenomena.
Plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

s,
algae Algae (; singular alga ) is an informal term for a large and diverse group of s. It is a grouping that includes species from multiple distinct s. Included organisms range from , such as '','' and the s, to forms, such as the , a large whi ...

and
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a of that obtain energy via . The name ''cyanobacteria'' refers to their color (), giving them their other name, "blue-green algae", though modern botanists restrict the term ' to s and do not ...

use
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regula ...

to
photosynthesize upright=1.8, Overall equation for the type of photosynthesis that occurs in plants Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Docto ...

carbohydrate is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common ex ...
from carbon dioxide and water, with oxygen produced as a waste product. CO2 is produced by all
aerobic organisms Image:Anaerobic.png, 300px, Aerobic and anaerobic bacteria can be identified by growing them in test tubes of thioglycollate broth: 1: Obligate aerobes need oxygen because they cannot ferment or respire anaerobically. They gather at the top of the ...
when they metabolize
organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
s to produce energy by
respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellular ...

. It is returned to water via the gills of fish and to the air via the lungs of air-breathing land animals, including humans. Carbon dioxide is produced during the processes of decay of organic materials and the
fermentation Fermentation is a metabolism, metabolic process that produces chemical changes in organic Substrate (chemistry), substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in ...

of sugars in
bread Bread is a staple food prepared from a dough of flour and water, usually by baking. Throughout recorded history, it has been a prominent food in large parts of the world. It is one of the oldest man-made foods, having been of significant impor ...

,
beer Beer is one of the oldest and most widely consumed alcoholic drink An alcoholic drink is a drink A drink (or beverage) is a liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continual ...

and
wine Wine is an alcoholic drink typically made from Fermentation in winemaking, fermented grapes. Yeast in winemaking, Yeast consumes the sugar in the grapes and converts it to ethanol and carbon dioxide, releasing heat in the process. Different ...

making. It is produced by combustion of
wood Wood is a porous and fibrous structural tissue found in the stems and roots of tree In botany, a tree is a perennial plant with an elongated Plant stem, stem, or trunk (botany), trunk, supporting branches and leaves in most species. ...

,
peat Peat (), also known as turf (), is an accumulation of partially decayed vegetation Vegetation is an assemblage of plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the ...
and other organic materials and
fossil fuel A fossil fuel is a -containing material formed underground from the remains of dead plants and animals that humans extract and to release for use. The main fossil s are , and , which humans extract through and . Fossil fuels may be burnt ...
s such as
coal Coal is a combustible black or brownish-black sedimentary rock, formed as stratum, rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen ...

,
petroleum Petroleum, also known as crude oil and oil, is a naturally occurring, yellowish-black liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isoc ...

and
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon In , a hydrocarbon is an consisting entirely of and . Hydrocarbons are examples of s. Hydrocarbons are generally colourless and hydrophobic wit ...

. It is an unwanted byproduct in many large scale
oxidation Redox (reduction–oxidation, pronunciation: or ) is a type of chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter ...

processes, for example, in the production of
acrylic acid Acrylic acid (IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Science ...

(over 5 million tons/year). It is a versatile industrial material, used, for example, as an inert gas in welding and
fire extinguishers A fire extinguisher is an active fire protection device used to extinguish or control small fires, often in emergency situations. It is not intended for use on an out-of-control fire, such as one which has reached the ceiling, endangers the use ...

, as a pressurizing gas in air guns and oil recovery, as a chemical feedstock and as a supercritical fluid solvent in decaffeination of coffee and
supercritical drying Supercritical drying, also known as critical point drying, is a process to remove liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an appl ...
. It is added to drinking water and
carbonated beverage A soft drink (see § Terminology for other names) is a drink that usually contains carbonated water (although some vitamin waters and lemonades are not carbonated), a sweetener, and a natural and/or artificial flavoring. The sweetener m ...
s including
beer Beer is one of the oldest and most widely consumed alcoholic drink An alcoholic drink is a drink A drink (or beverage) is a liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continual ...

and
sparkling wine Sparkling wine is a wine Wine is an alcoholic drink typically made from fermented grape juice. Yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hun ...
effervescence Image:Soda bubbles macro.jpg, Bubbles of carbon dioxide float to the surface of a Soft drink#Carbonated drinks, carbonated soft drink. Effervescence is the escape of gas from an aqueous solution and the foaming or fizzing that results from that rel ...
. The frozen solid form of CO2, known as ''
dry ice Dry ice is the solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is chara ...

'' is used as a refrigerant and as an abrasive in
dry-ice blasting Dry ice blasting is a form of carbon dioxide cleaning, where dry ice, the solid form of carbon dioxide Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% higher than that of dry air. Carbon dioxide molecules con ...
. It is a feedstock for the synthesis of fuels and chemicals. Carbon dioxide is the most significant long-lived
greenhouse gas A greenhouse gas (GHG or GhG) is a gas Gas is one of the four fundamental states of matter (the others being solid, liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformat ...
in
Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmosphere (0.04402961% at April 2019 concentration ). Number ...
. Since the
Industrial Revolution The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe Continental Europe or mainland Europe is the contiguous continent A continent is any of several large landmasse ...
anthropogenic emissions – primarily from use of fossil fuels and
deforestation deforestation in 1750-2004 (net loss) showing anthropogenic modification of remaining forest. File:MODIS (2020-08-01).jpg, 300px, Dry seasons, exacerbated by climate change, and the use of slash-and-burn methods for clearing tropical forest ...

– have rapidly increased its concentration in the atmosphere, leading to
global warming Contemporary climate change includes both the global warming caused by humans, and its impacts on Earth's weather patterns. There have been , but the current changes are more rapid than any known events in Earth's history. The main cau ...

. Carbon dioxide also causes
ocean acidification Ocean acidification is the ongoing decrease in the pH value of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of cont ...
because it dissolves in water to form
carbonic acid In chemistry, carbonic acid is a dibasic acid with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical ...

.

# History

Carbon dioxide was the first gas to be described as a discrete substance. In about 1640, the
Flemish Flemish (''Vlaams'') is a Low Franconian Low Franconian, Low Frankish, NetherlandicSarah Grey Thomason, Terrence Kaufman: ''Language Contact, Creolization, and Genetic Linguistics'', University of California Press, 1991, p. 321. (Calling it " ...
chemist
Jan Baptist van Helmont Jan Baptist van Helmont (; ; 12 January 1580 – 30 December 1644) was a chemist, physiologist, and physician A physician (American English), medical practitioner (English in the Commonwealth of Nations, Commonwealth English), medic ...

observed that when he burned
charcoal or soil, and firing it (circa 1890) Charcoal is a lightweight black carbon Carbon (from la, carbo "coal") is a chemical element with the Symbol (chemistry), symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent ...

in a closed vessel, the mass of the resulting
ash Ash or ashes are the solid remnants of fire BBQ. Fire is the rapid oxidation of a material in the exothermic chemical process of combustion, releasing heat, light, and various reaction Product (chemistry), products. Fire is hot because th ...
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 sylvestris''). The properties of carbon dioxide were further studied in the 1750s by the
Scottish Scottish usually refers to something of, from, or related to Scotland, including: *Scottish Gaelic, a Celtic Goidelic language of the Indo-European language family native to Scotland *Scottish English *Scottish national identity, the Scottish iden ...

physician
Joseph Black Joseph Black (16 April 1728 – 6 December 1799) was a Scottish physicist and chemist, known for his discoveries of magnesium, latent heat, specific heat, and carbon dioxide. He was Professor of Anatomy and Chemistry at the University of Glasgo ...
. He found that
limestone Limestone is a common type of carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its nat ...

(
calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together ...

) could be heated or treated with
acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of forming a with an (a ). The first category of acids are the proton donors, or s. In the special case of , proton donors form the H3O+ and are ...
s 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
limewater Limewater is the common name for a dilute aqueous solution of calcium hydroxide. Calcium hydroxide, Ca(OH)2, is sparsely soluble at room temperature in water (1.5 g/L at 25 °C). "Pure" (i.e. less than or fully saturated) limewater is cl ...

(a saturated aqueous solution of
calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula calcium, Ca(Hydroxide, OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaking (ge ...

), it would
precipitate Precipitation is the process of conversion of a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All ever ...
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 Joseph Priestley (; 24 March 1733 – 6 February 1804) was an English Chemistry, chemist, Natural philosophy, natural philosopher, English Separatist, separatist theologian, Linguist, grammarian, multi-subject educator, and Liberalism, liber ...
published a paper entitled ''Impregnating Water with Fixed Air'' in which he described a process of dripping
sulfuric acid Sulfuric acid ( and the ) or sulphuric acid (), known in antiquity as oil of vitriol, is a composed of the elements , and , with the . It is a colorless, odorless and liquid that is with water. Pure sulfuric acid does not exist naturally ...

(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. Carbon dioxide was first liquefied (at elevated pressures) in 1823 by
Humphry Davy Sir Humphry Davy, 1st Baronet (17 December 177829 May 1829) was a Cornish people, Cornish chemist and inventor who invented the Davy lamp and a very early form of arc lamp. He is also remembered for isolating, by using electricity, a series of ...

and
Michael Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge ...

. The earliest description of solid carbon dioxide (
dry ice Dry ice is the solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is chara ...

) was given by the French inventor Adrien-Jean-Pierre Thilorier, who in 1835 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.

# Chemical and physical properties

## Structure and bonding

The carbon dioxide molecule is linear and
centrosymmetric In crystallography Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids (see crystal structure). The word "crystallography" is derived from the Greek words ''crystallon'' "cold drop, frozen ...
at equilibrium. The carbon–oxygen bond length is 116.3  pm, noticeably shorter than the
bond length Bond or bonds may refer to: Common meanings * Bond (finance) In finance Finance is the study of financial institutions, financial markets and how they operate within the financial system. It is concerned with the creation and management of ...
of a C–O single bond and even shorter than most other C–O multiply-bonded functional groups. Since it is centrosymmetric, the molecule has no electrical
dipole In electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is c ...

. As a linear triatomic molecule, CO2 has four vibrational modes as shown in the diagram. However, the symmetric stretching mode does not create a dipole and so is not observed in the IR spectrum. The two bending modes are degenerate, meaning that they correspond to only one frequency. Consequently, only two vibrational bands are observed in the
IR spectrum Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption spectroscopy, absorption, emission spectrum, emission, or reflection (physics), reflection. I ...
– an antisymmetric stretching mode at
wavenumber In the physical science Physical science is a branch of natural science that studies non-living systems, in contrast to life science. It in turn has many branches, each referred to as a "physical science", together called the "physical scienc ...
2349 cm−1 (wavelength 4.25 μm) and a
degenerate Degeneracy may refer to: Science * Codon degeneracy * Degeneracy (biology), the ability of elements that are structurally different to perform the same function or yield the same output * Degeneration (medical) ** Degenerative disease, a diseas ...

pair of bending modes at 667 cm−1 (wavelength 15 μm). There is also a symmetric stretching mode at 1388 cm−1 which is only observed in the
Raman spectrumImage:Raman energy levels.svg, 450px, Energy-level diagram showing the states involved in Raman spectra. Raman spectroscopy (); (named after Indian physicist C. V. Raman) is a Spectroscopy, spectroscopic technique typically used to determine vibrati ...
. As a result of the two bending modes, the molecule is only strictly linear when the amount of bending is zero. It has been shown both by theory and by Coulomb explosion imaging experiments that this is never actually true for both modes at once. In a gas phase sample of carbon dioxide, none of the molecules are linear as a result of the vibrational motions. However, the molecular geometry is still described as linear, which describes the average atomic positions corresponding to minimum potential energy. This is also true for other “linear” molecules.

## In aqueous solution

Carbon dioxide is
soluble Solubility is the property of a solid, liquid or gaseous chemical substance called ''solution, solute'' to dissolve in a solid, liquid or gaseous solvent. The solubility of a substance fundamentally depends on the Physical property, physical an ...
in water, in which it reversibly forms (
carbonic acid In chemistry, carbonic acid is a dibasic acid with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical ...

), which is a
weak acid Acid strength is the tendency of an acid An acid is a molecule or ion capable of donating a proton (hydrogen ion H+) (a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid), or, alternatively, capable of forming a covalent bond wi ...

since its ionization in water is incomplete. : + The hydration equilibrium constant of carbonic acid is $K_=\frac=1.70\times 10^$ (at 25 °C). Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as CO2 molecules, not affecting the pH. The relative concentrations of , and the forms (
bicarbonate In inorganic chemistry#REDIRECT Inorganic chemistry features unusual bonding B: Caesium chloride Caesium chloride or cesium chloride is the inorganic compound with the formula Caesium, CsChloride, Cl. This colorless salt is an important sourc ...

) and (
carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its natural form as a crystallinity, crystalline min ...

) depend on the . As shown in a
Bjerrum plotBjerrum may refer to: ;People * Niels Janniksen Bjerrum (1879 – 1958), was a Danish chemist (son of Jannik Petersen Bjerrum and father of Jannik Bjerrum). * Jannik Bjerrum (1909 – 1992), was a Danish chemist (son of Niels Janniksen Bjerrum). * ...
, 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 acid, diprotic, carbonic acid has two acid dissociation constants, the first one for the dissociation into the bicarbonate (also called hydrogen carbonate) ion (HCO3): :H2CO3 HCO3 + H+ :''K''a1 = ; p''K''a1 = 3.6 at 25 °C. This is the ''true'' first acid dissociation constant, defined as $K_=\frac$, where the denominator includes only covalently bound H2CO3 and does not include hydrated CO2(aq). The much smaller and often-quoted value near is an ''apparent'' value calculated on the (incorrect) assumption that all dissolved CO2 is present as carbonic acid, so that $K_=\frac$. Since most of the dissolved CO2 remains as CO2 molecules, ''K''a1(apparent) has a much larger denominator and a much smaller value than the true ''K''a1. The
bicarbonate In inorganic chemistry#REDIRECT Inorganic chemistry features unusual bonding B: Caesium chloride Caesium chloride or cesium chloride is the inorganic compound with the formula Caesium, CsChloride, Cl. This colorless salt is an important sourc ...

ion is an amphoteric species that can act as an acid or as a base, depending on pH of the solution. At high , it dissociates significantly into the
carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its natural form as a crystallinity, crystalline min ...

ion (CO32−): :HCO3 CO32− + H+ :''K''a2 = ; p''K''a2 = 10.329 In organisms carbonic acid production is catalysed by the enzyme, carbonic anhydrase.

## Chemical reactions of CO2

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 CO2 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 CO2 to give carboxylates: :MR + CO2 → RCO2M :where M = Lithium, Li or Magnesium, Mg Bromine, Br and R = alkyl or aryl. In metal carbon dioxide complexes, CO2 serves as a ligand, which can facilitate the conversion of CO2 to other chemicals. The reduction of CO2 to Carbon monoxide, CO is ordinarily a difficult and slow reaction: :CO2 + 2 e + 2H+ → CO + H2O Photoautotrophs (i.e. plants and
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a of that obtain energy via . The name ''cyanobacteria'' refers to their color (), giving them their other name, "blue-green algae", though modern botanists restrict the term ' to s and do not ...

) use the energy contained in sunlight to Photosynthesis, photosynthesize simple sugars from CO2 absorbed from the air and water: : ''n'' CO2 + ''n'' → + ''n'' 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 conditions for temperature and pressure, standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m3, about 1.53 times that of Earth's atmosphere, air. Carbon dioxide has no liquid state at pressures below . At 1 atmosphere (near mean sea level pressure), the gas deposition (physics), deposits directly to a solid at temperatures below and the solid sublimation (chemistry), sublimes directly to a gas above −78.5 °C. In its solid state, carbon dioxide is commonly called
dry ice Dry ice is the solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is chara ...

. Liquid carbon dioxide forms only at pressures above 5.1 atm; the triple point of carbon dioxide is about 5.1 bar (unit), bar (517 kPa) at 217 K (see phase diagram). The Critical point (thermodynamics), critical point is 7.38 MPa at 31.1 °C. Another form of solid carbon dioxide observed at high pressure is an amorphous glass-like solid. This form of glass, called ''amorphous carbonia, carbonia'', is 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, 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.

# Isolation and production

Carbon dioxide can be obtained by distillation from air, but the method is inefficient. Industrially, carbon dioxide is predominantly an unrecovered waste product, produced by several methods which may be practiced at various scales. The combustion of all carbon-based fuels, such as methane (
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon In , a hydrocarbon is an consisting entirely of and . Hydrocarbons are examples of s. Hydrocarbons are generally colourless and hydrophobic wit ...

), petroleum distillates (gasoline, Diesel fuel, diesel, kerosene, propane), coal, wood and generic organic matter produces carbon dioxide and, except in the case of pure carbon, water. As an example, the chemical reaction between methane and oxygen: : + 2 → + 2 It is produced by thermal decomposition of limestone, by heating (calcining) at about , in the manufacture of Calcium oxide, quicklime (calcium oxide, ), a compound that has many industrial uses: : → + Iron is reduced from its oxides with coke (fuel), coke in a blast furnace, producing pig iron and carbon dioxide: Carbon dioxide is a byproduct of the industrial production of hydrogen by steam reforming and the water gas shift reaction in ammonia production. These processes begin with the reaction of water and natural gas (mainly methane). This is a major source of food-grade carbon dioxide for use in carbonation of
beer Beer is one of the oldest and most widely consumed alcoholic drink An alcoholic drink is a drink A drink (or beverage) is a liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continual ...

and soft drinks, and is also used for stunning animals such as poultry. In the summer of 2018 a shortage of carbon dioxide for these purposes arose in Europe due to the temporary shut-down of several ammonia plants for maintenance. Acids liberate CO2 from most metal carbonates. Consequently, it may be obtained directly from natural carbon dioxide spring (hydrosphere), springs, where it is produced by the action of acidified water on
limestone Limestone is a common type of carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its nat ...

or Dolomite (mineral), dolomite. The reaction between hydrochloric acid and calcium carbonate (limestone or chalk) is shown below: : + 2 → + The
carbonic acid In chemistry, carbonic acid is a dibasic acid with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical ...

() then decomposes to water and CO2: : → + Such reactions are accompanied by foaming or bubbling, or both, as the gas is released. They have widespread uses in industry because they can be used to neutralize waste acid streams. Carbon dioxide is a by-product of the Fermentation (biochemistry), fermentation of sugar in the brewing of
beer Beer is one of the oldest and most widely consumed alcoholic drink An alcoholic drink is a drink A drink (or beverage) is a liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continual ...

, whisky and other alcoholic beverages and in the production of bioethanol. Yeast metabolizes sugar to produce CO2 and ethanol, also known as alcohol, as follows: : → 2 + 2 All cellular respiration, aerobic organisms produce CO2 when they oxidize
carbohydrate is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common ex ...
s, fatty acids, and proteins. The large number of reactions involved are exceedingly complex and not described easily. Refer to (cellular respiration, anaerobic respiration and photosynthesis). The equation for the respiration of glucose and other monosaccharides is: : + 6 → 6 + 6 Anaerobic organisms decompose organic material producing methane and carbon dioxide together with traces of other compounds. Regardless of the type of organic material, the production of gases follows well defined chemical kinetics, kinetic pattern. Carbon dioxide comprises about 40–45% of the gas that emanates from decomposition in landfills (termed "landfill gas"). Most of the remaining 50–55% is methane.

# Applications

Carbon dioxide is used by the food industry, the oil industry, and the chemical industry. The compound has varied commercial uses but one of its greatest uses as a chemical is in the production of carbonated beverages; it provides the sparkle in carbonated beverages such as soda water, beer and sparkling wine.

## Precursor to chemicals

In the chemical industry, carbon dioxide is mainly consumed as an ingredient in the production of urea, with a smaller fraction being used to produce methanol and a range of other products. Some carboxylic acid derivatives such as sodium salicylate are prepared using CO2 by the Kolbe-Schmitt reaction. In addition to conventional processes using CO2 for chemical production, electrochemical methods are also being explored at a research level. In particular, the use of renewable energy for production of fuels from CO2 (such as methanol) is attractive as this could result in fuels that could be easily transported and used within conventional combustion technologies but have no net CO2 emissions.

## 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 (listed as E number E290), US and Australia and New Zealand (listed by its INS number 290). A candy called Pop Rocks is pressurized with carbon dioxide gas at about . When placed in the mouth, it dissolves (just like other hard candy) and releases the gas bubbles with an audible pop. Leavening agents cause dough to rise by producing carbon dioxide. 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
acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of forming a with an (a ). The first category of acids are the proton donors, or s. In the special case of , proton donors form the H3O+ and are ...
s.

### Beverages

Carbon dioxide is used to produce carbonation, carbonated soft drinks and soda water. Traditionally, the carbonation of 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, the most common method used is carbonation with recycled carbon dioxide. With the exception of British cask ale#Real ale, 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. The taste of soda water (and related taste sensations in other carbonated beverages) is an effect of the dissolved carbon dioxide rather than the bursting bubbles of the gas. Carbonic anhydrase 4 converts to
carbonic acid In chemistry, carbonic acid is a dibasic acid with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical ...

leading to a sour taste, and also the dissolved carbon dioxide induces a somatosensory response.

### Winemaking

Carbon dioxide in the form of
dry ice Dry ice is the solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is chara ...

is often used during the cold soak phase in winemaking to cool clusters of grapes quickly after picking to help prevent spontaneous Fermentation (wine), fermentation by wild yeast (wine), yeast. The main advantage of using dry ice over water ice is that it cools the grapes without adding any additional water that might decrease the sugar concentration in the grape must, and thus the ethanol, alcohol concentration in the finished wine. 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 (wine), 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.

### Stunning animals

Carbon dioxide is often used to "stun" animals before slaughter. "Stunning" may be a misnomer, as the animals are not knocked out immediately and may suffer distress.

## Inert gas

It is one of the most commonly used compressed gases for pneumatic (pressurized gas) systems in portable pressure tools. Carbon dioxide is also used as an atmosphere for welding, although in the welding arc, it reacts to oxidation, 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. 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. 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 , 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. Aluminium capsules of CO2 are also sold as supplies of compressed gas for air guns, paintball markers/guns, inflating bicycle tires, and for making carbonated water. High concentrations of carbon dioxide can also be used to kill pests. Liquid carbon dioxide is used in
supercritical drying Supercritical drying, also known as critical point drying, is a process to remove liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an appl ...
of some food products and technological materials, in the preparation of specimens for scanning electron microscopy and in the decaffeination of coffee beans.

## Fire extinguisher

Carbon dioxide can be used to extinguish flames by flooding the environment around the flame with the gas. It does not itself react to extinguish the flame, but starves the flame of oxygen by displacing it. Some Fire extinguisher#Clean agents and carbon dioxide, 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, because although it excludes oxygen, it does not cool the burning substances significantly and when the carbon dioxide disperses they are free to catch fire upon exposure to atmospheric oxygen. Their desirability in electrical fire stems from the fact that, unlike water or other chemical based methods, Carbon dioxide will not cause short circuits, leading to even more damage to equipment. Because it is a gas, it is also easy to dispense large amounts of the gas automatically in IT infrastructure rooms, where the fire itself might be hard to reach with more immediate methods because it is behind rack doors and inside of cases. 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. 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 can cause suffocation in sufficiently high concentrations. A review of CO2 systems identified 51 incidents between 1975 and the date of the report (2000), causing 72 deaths and 145 injuries.

## Supercritical CO2 as solvent

Liquid carbon dioxide is a good solvent for many lipophilic
organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
s 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 also used by some dry cleaning, dry cleaners for this reason (see green chemistry). It is used in the preparation of some Aerogel#Production, aerogels because of the properties of supercritical carbon dioxide.

## Agriculture

Plants require carbon dioxide to conduct photosynthesis. The atmospheres of greenhouses may (if of large size, must) be enriched with additional CO2 to sustain and increase the rate of plant growth. At very high concentrations (100 times atmospheric concentration, or greater), 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 whitefly, whiteflies and spider mites in a greenhouse.

## Medical and pharmacological uses

In medicine, up to 5% carbon dioxide (130 times atmospheric concentration) is added to
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

for stimulation of breathing after apnea and to stabilize the balance in blood. Carbon dioxide can be mixed with up to 50% oxygen, forming an inhalable gas; this is known as Carbogen and has a variety of medical and research uses.

## Energy

### Fossil fuel recovery

Carbon dioxide is used in enhanced oil recovery where it is injected into or adjacent to producing oil wells, usually under Supercritical fluid, supercritical conditions, when it becomes miscibility, miscible with the oil. This approach can increase original oil recovery by reducing residual oil saturation by between 7% to 23% additional to Extraction of petroleum#Primary recovery, primary extraction. It acts as both a pressurizing agent and, when dissolved into the underground crude oil, significantly reduces its viscosity, and changing surface chemistry enabling the oil to flow more rapidly through the reservoir to the removal well. In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points. In enhanced coal bed methane recovery, carbon dioxide would be pumped into the coal seam to displace methane, as opposed to current methods which primarily rely on the removal of water (to reduce pressure) to make the coal seam release its trapped methane.

### Bio transformation into fuel

It has been proposed that CO2 from power generation be bubbled into ponds to stimulate growth of algae that could then be converted into biodiesel fuel. A strain of the cyanobacterium ''Synechococcus, Synechococcus elongatus'' has been genetically engineered to produce the fuels isobutyraldehyde and isobutanol from CO2 using photosynthesis.

## 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 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 Dichlorodifluoromethane, R-12 and may enjoy a renaissance due to the fact that R134a contributes to climate change more than CO2 does. Its physical properties are highly favorable for cooling, refrigeration, and heating purposes, having a high volumetric cooling capacity. Due to the need to operate at pressures of up to , 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 R134a. Its environmental advantages (Global warming potential, GWP of 1, non-ozone depleting, non-toxic, non-flammable) could make it the future working fluid to replace current HFCs in cars, supermarkets, and heat pump water heaters, among others. Coca-Cola has fielded CO2-based beverage coolers and the United States Army, U.S. Army is interested in CO2 refrigeration and heating technology. The global automobile industry is expected to decide on the next-generation refrigerant in car air conditioning. CO2 is one discussed option.(see Sustainable automotive air conditioning)

## Minor uses

Carbon dioxide is the active laser medium, lasing medium in a carbon dioxide laser, which is one of the earliest type of lasers. Carbon dioxide can be used as a means of controlling the of swimming pools, by continuously adding gas to the water, thus keeping the pH from rising. Among the advantages of this is the avoidance of handling (more hazardous) acids. Similarly, it is also used in the maintaining Reef aquarium, reef aquaria, where it is commonly used in calcium reactors to temporarily lower the pH of water being passed over
calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together ...

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. Used as the primary coolant in the British advanced gas-cooled reactor for nuclear power generation. Carbon dioxide induction is commonly used for the euthanasia of laboratory research animals. Methods to administer CO2 include placing animals directly into a closed, prefilled chamber containing CO2, or exposure to a gradually increasing concentration of CO2. In 2013, the American Veterinary Medical Association issued new guidelines for carbon dioxide induction, stating that a displacement rate of 30% to 70% of the gas chamber volume per minute is optimal for the humane euthanization of small rodents. However, there is opposition to the practice of using carbon dioxide for this, on the grounds that it is cruel. Carbon dioxide is also used in several related carbon dioxide cleaning, cleaning and surface preparation techniques.

# In Earth's atmosphere

Carbon dioxide in Earth's atmosphere is a
trace gas#REDIRECT Trace gas Trace gases are those gases in the atmosphere other than nitrogen Nitrogen is the chemical element with the Symbol (chemistry), symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel ...
, having a global average concentration of 415 parts per million by volume (or 630 parts per million by mass) as of the end of year 2020.[ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_mm_gl.txt National Oceanic & Atmospheric Administration (NOAA) – Earth System Research Laboratory (ESRL), Trends in Carbon Dioxide: Globally averaged marine surface monthly mean data] Values given are dry air mole fractions expressed in parts per million ( ppm). For an ideal gas mixture this is equivalent to parts per million by volume (ppmv). Atmospheric concentrations fluctuate slightly with the seasons, falling during the Northern Hemisphere spring and summer as plants consume the gas and rising during northern autumn and winter as plants go dormant or die and decay. Concentrations also vary on a regional basis, most strongly planetary boundary layer, near the ground with much smaller variations aloft. In urban areas concentrations are generally higher and indoors they can reach 10 times background levels. The concentration of carbon dioxide has risen due to human activities. The extraction and burning of
fossil fuel A fossil fuel is a -containing material formed underground from the remains of dead plants and animals that humans extract and to release for use. The main fossil s are , and , which humans extract through and . Fossil fuels may be burnt ...
s, using carbon that has been sequestered for many millions of years in the lithosphere, has caused the atmospheric concentration of to increase by about 50% since the beginning of the Industrial Revolution, age of industrialization up to year 2020.Friedlingstein, P., Jones, M., O'Sullivan, M., Andrew, R., Hauck, J., Peters, G., Peters, W., Pongratz, J., Sitch, S., Le Quéré, C. and 66 others (2019) "Global carbon budget 2019". ''Earth System Science Data'', 11(4): 1783–1838. . Most from human activities is released from burning coal, petroleum, and natural gas. Other large anthropogenic sources include cement production,
deforestation deforestation in 1750-2004 (net loss) showing anthropogenic modification of remaining forest. File:MODIS (2020-08-01).jpg, 300px, Dry seasons, exacerbated by climate change, and the use of slash-and-burn methods for clearing tropical forest ...

, and biomass burning. Human activities emit over 30 billion tons of (9 billion tons of fossil carbon) per year, while volcanoes emit only between 0.2 and 0.3 billion tons of . Human activities have caused CO2 to increase above levels not seen in hundreds of thousands of years. Currently, about half of the carbon dioxide released from the Global warming, burning of fossil fuels remains in the atmosphere and is not absorbed by vegetation and the oceans. While transparent to visible light, carbon dioxide is a
greenhouse gas A greenhouse gas (GHG or GhG) is a gas Gas is one of the four fundamental states of matter (the others being solid, liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformat ...
, absorbing and emitting infrared radiation at its two infrared-active vibrational frequencies (see the section "Carbon dioxide#Structure and bonding, Structure and bonding" above). Light emission from the earth's surface is most intense in the infrared region between 200 and 2500 cm−1, as opposed to light emission from the much hotter sun which is most intense in the visible region. Absorption of infrared light at the vibrational frequencies of atmospheric traps energy near the surface, warming the surface and the lower atmosphere. Less energy reaches the upper atmosphere, which is therefore cooler because of this absorption. Increases in atmospheric concentrations of and other long-lived greenhouse gases such as methane, nitrous oxide and ozone have correspondingly strengthened their absorption and emission of infrared radiation, causing the rise in average global temperature since the mid-20th century. Carbon dioxide is of greatest concern because it exerts a larger overall warming influence than all of these other gases combined. It furthermore has an Greenhouse gas#Atmospheric lifetime, atmospheric lifetime that increases with the cumulative amount of fossil carbon extracted and burned, due to the imbalance that this activity has imposed on Earth's fast carbon cycle. This means that some fraction (a projected 20-35%) of the fossil carbon transferred thus far will in-effect persist in the atmosphere as elevated levels for many thousands of years after these carbon transfer activities begin to subside. Not only do increasing concentrations lead to increases in global surface temperature, but increasing global temperatures also cause increasing concentrations of carbon dioxide. This produces a positive feedback for changes induced by other processes such as Milankovitch cycles, orbital cycles. Five hundred million years ago the concentration was 20 times greater than today, decreasing to 4–5 times during the Jurassic period and then slowly declining with Azolla Event, a particularly swift reduction occurring 49 million years ago. Local concentrations of carbon dioxide can reach high values near strong sources, especially those that are isolated by surrounding terrain. At the Bossoleto hot spring near Rapolano Terme in Tuscany, Italy, situated in a bowl-shaped depression about in diameter, concentrations of CO2 rise to above 75% overnight, sufficient to kill insects and small animals. After sunrise the gas is dispersed by convection. 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.

# In the oceans

Carbon dioxide dissolves in the ocean to form
carbonic acid In chemistry, carbonic acid is a dibasic acid with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical ...

(H2CO3),
bicarbonate In inorganic chemistry#REDIRECT Inorganic chemistry features unusual bonding B: Caesium chloride Caesium chloride or cesium chloride is the inorganic compound with the formula Caesium, CsChloride, Cl. This colorless salt is an important sourc ...

(HCO3) and
carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its natural form as a crystallinity, crystalline min ...

(CO32−). There is about fifty times as much carbon dioxide dissolved in the oceans 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. As the concentration of carbon dioxide increases in the atmosphere, the increased uptake of carbon dioxide into the oceans is causing a measurable decrease in the pH of the oceans, which is referred to as
ocean acidification Ocean acidification is the ongoing decrease in the pH value of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of cont ...
. This reduction in pH affects biological systems in the oceans, primarily oceanic calcification, calcifying organisms. These effects span the food chain from autotrophs to heterotrophs and include organisms such as coccolithophores, corals, foraminifera, echinoderms, crustaceans and mollusca, mollusks. Under normal conditions, calcium carbonate is stable in surface waters since the carbonate ion is at supersaturation, supersaturating concentrations. However, as ocean pH falls, so does the concentration of this ion, and when carbonate becomes undersaturated, structures made of calcium carbonate are vulnerable to dissolution. Corals, coccolithophore algae, coralline algae, foraminifera, shellfish and pteropods experience reduced calcification or enhanced dissolution when exposed to elevated . 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) 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, forms carbonic acid in equilibrium with bicarbonate. Some of these chemical species are consumed by photosynthetic organisms that remove carbon from the 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, although there's evidence of increased shell production by certain species under increased CO2 content. NOAA states in their May 2008 "State of the science fact sheet for
ocean acidification Ocean acidification is the ongoing decrease in the pH value of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of cont ...
" 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:
"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 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. Carbon dioxide is also introduced into the oceans through hydrothermal vents. The ''Champagne'' hydrothermal vent, found at the Northwest Eifuku volcano in the Marianas Trench, produces almost pure liquid carbon dioxide, one of only two known sites in the world as of 2004, the other being in the Okinawa Trough. The finding of a submarine lake of liquid carbon dioxide in the Okinawa Trough was reported in 2006.

# 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 Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

as part of their metabolism. This includes all plants, algae and animals and aerobic respiration, 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, compensation point, 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 Algae (; singular alga ) is an informal term for a large and diverse group of s. It is a grouping that includes species from multiple distinct s. Included organisms range from , such as '','' and the s, to forms, such as the , a large whi ...

and (
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a of that obtain energy via . The name ''cyanobacteria'' refers to their color (), giving them their other name, "blue-green algae", though modern botanists restrict the term ' to s and do not ...

) into fuel, energy-rich organic
molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion I ...

s such as glucose, thus creating their own food by photosynthesis. Photosynthesis uses carbon dioxide and water to produce sugars from which other
organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
s can be constructed, and
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

is produced as a by-product. RuBisCO, 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 Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together ...

scales. A globally significant species of coccolithophore is ''Emiliania huxleyi'' whose calcite scales have formed the basis of many sedimentary rocks such as
limestone Limestone is a common type of carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its nat ...

, 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 Free-Air Concentration Enrichment, 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, 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 Asphyxiant gas, 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 Mt. Nyiragongo. The Swahili language, Swahili term for this phenomenon is 'mazuku'. Adaptation to increased concentrations of CO2 occurs in humans, including Respiratory adaptation, 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 acidosis, 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.

### 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). 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 Domestic Canary, 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 could also detect high levels of blackdamp (which sinks, and collects near the floor) by burning less brightly, while methane, another suffocating gas and explosion risk, would make the lamp burn more brightly. In February 2020, three people died from suffocation at a party in Moscow when dry ice (frozen CO2) was added to a swimming pool to cool it down.

## Human physiology

### Content

The body produces approximately of carbon dioxide per day per person, containing of carbon. In humans, this carbon dioxide is carried through the venous system and is breathed out through the lungs, resulting in lower concentrations in the arteries. The carbon dioxide content of the blood is often given as the partial pressure, which is the pressure which carbon dioxide would have had if it alone occupied the volume. In humans, the blood carbon dioxide contents is shown in the adjacent table:

### Transport in the blood

CO2 is carried in blood in three different ways. (The exact percentages vary depending whether it is arterial or venous blood). * Most of it (about 70% to 80%) is converted to
bicarbonate In inorganic chemistry#REDIRECT Inorganic chemistry features unusual bonding B: Caesium chloride Caesium chloride or cesium chloride is the inorganic compound with the formula Caesium, CsChloride, Cl. This colorless salt is an important sourc ...

ions by the enzyme carbonic anhydrase in the red blood cells, by the reaction CO2 + → → + . * 5–10% is dissolved in the Blood plasma, plasma * 5–10% is bound to hemoglobin as carbamino compounds Hemoglobin, the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbon dioxide. However, the CO2 bound to hemoglobin does not bind to the same site as oxygen. Instead, it combines with the N-terminal groups on the four globin chains. However, because of allosteric regulation, allosteric effects on the hemoglobin molecule, the binding of CO2 decreases the amount of oxygen that is bound for a given partial pressure of oxygen. This is known as the Haldane Effect, and is important in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure of CO2 or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the Bohr effect.

### Regulation of respiration

Carbon dioxide is one of the mediators of local autoregulation of blood supply. If its concentration is high, the capillaries expand to allow a greater blood flow to that tissue. Bicarbonate ions are crucial for regulating blood pH. A person's breathing rate influences the level of CO2 in their blood. Breathing that is too slow or shallow causes respiratory acidosis, while breathing that is too rapid leads to hyperventilation, which can cause alkalosis, respiratory alkalosis. Although the body requires oxygen for metabolism, low oxygen levels normally do not stimulate breathing. Rather, breathing is stimulated by higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (such as pure nitrogen) can lead to loss of consciousness without ever experiencing air hunger. This is especially perilous for high-altitude fighter pilots. It is also why flight attendants instruct passengers, in case of loss of cabin pressure, to apply the oxygen mask to themselves first before helping others; otherwise, one risks losing consciousness. The respiratory centers try to maintain an arterial CO2 pressure of 40 mm Hg. With intentional hyperventilation, the CO2 content of arterial blood may be lowered to 10–20 mm Hg (the oxygen content of the blood is little affected), and the respiratory drive is diminished. This is why one can hold one's breath longer after hyperventilating than without hyperventilating. This carries the risk that unconsciousness may result before the need to breathe becomes overwhelming, which is why hyperventilation is particularly dangerous before free diving.

* * * * (from the atmosphere) * * * * s * * * * List of least carbon efficient power stations * List of countries by carbon dioxide emissions * * * (early work on CO2 and climate change) * * NASA's *

# References

* * * *

*

by Amerigas.
CDC – NIOSH Pocket Guide to Chemical Hazards – Carbon Dioxide

Trends in Atmospheric Carbon Dioxide
(NOAA)
"A War Gas That Saves Lives"
''Popular Science'', June 1942, pp. 53–57.
Reactions, Thermochemistry, Uses, and Function of Carbon Dioxide

an

at ''The Periodic Table of Videos'' (University of Nottingham) {{DEFAULTSORT:Carbon Dioxide Carbon dioxide, Acid anhydrides Acidic oxides Coolants Fire suppression agents Greenhouse gases Household chemicals Inorganic solvents Laser gain media Nuclear reactor coolants Oxocarbons Propellants Refrigerants Gaseous signaling molecules E-number additives