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Carbon Dioxide

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Carbon dioxide, a.k.a. CO2, is a chemical compound composed of two Oxygen Atoms covalently bonded to a single Carbon atom. A gas at standard temperature and pressure that exists in Earth's atmosphere in this state, CO2 is currently at a globally averaged concentration of approximately 387 ppm by Volume in the Earth's atmosphere, although this varies both by location and time. Carbon dioxide is an important Greenhouse gas because it transmits visible light but absorbs strongly in the Infrared.

Carbon dioxide is produced by all animals, plants, fungi and microorganisms during Respiration and is used by plants during PH. This is to make sugars which may either be consumed again in respiration or used as the raw material for plant growth. It is, therefore, a major component of the Carbon. Carbon dioxide is generated as a byproduct of the combustion of Fossil fuel or vegetable matter, among other chemical processes. Inorganic carbon dioxide is output by Volcanoes and other Geothermal processes such as Hot springs.

Carbon dioxide has no liquid state at pressures below 5.1 Atmosphere (unit), but is a solid at temperatures below -78 °C. In its solid state, carbon dioxide is commonly called Dry ice.

Chemical and physical propertiesEdit

File:Carbon dioxide pressure-temperature phase diagram.jpg
Carbon dioxide pressure-temperature phase diagram showing the Triple point of carbon dioxide

Carbon dioxide is a colorless, odorless gas. 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 Carbon. This sensation can also occur during an attempt to stifle a burp after drinking a Carbon. 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.

At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m³, about 1.5 times that of Earth's atmosphere. The carbon dioxide molecule (O=C=O) contains two double covalent bonds and has a linear shape. Its dipole moment is laterally balanced, and as it is fully oxidized, it is moderately reactive and is non-flammable, but will support the combustion of metals such as Magnesium.

File:Dry Ice Pellets Subliming.jpg
Small pellets of dry ice subliming in air.
Crystal structure of dry ice

At −78.51° C or -109.3° F, carbon dioxide changes directly from a solid phase to a gaseous phase through Sublimation (chemistry), or from gaseous to solid through Deposition (chemistry). Solid carbon dioxide is normally 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 it is also used in the shipping industry. The largest non-cooling use for dry ice is Dry ice.

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 (thermodynamics) is 7.38 MPa at 31.1 °C.

An alternative form of solid carbon dioxide, an Amorphous glass-like form, is possible, although not at atmospheric pressure. This form of glass, called Amorphous, was produced by Supercooling heated 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 of human understandingEdit

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).

The properties of carbon dioxide were studied more thoroughly in the 1750s by the Scottish physician Joseph Black. He found that Lime (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 did not support either flame or animal life. He also found that when bubbled through an aqueous solution of lime (Calcium hydroxide), it would Precipitation (chemistry) 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.

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and Michael Faraday. 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 .


Carbon dioxide may be obtained from air Distillation. However, this yields only very small quantities of . A large variety of chemical reactions yield carbon dioxide, such as the reaction between most acids and most metal carbonates. For example, the reaction between Sulfuric acid and calcium carbonate (limestone or chalk) is depicted below:

The then decomposes to water and . 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 :

The Combustion of all carbon containing fuels, such as Methane (Natural gas), petroleum distillates (Gas, Diesel, Kerosene, Propane), but also of coal and wood, will yield carbon dioxide and water. As an example the chemical reaction between methane and oxygen is given below.

Iron is reduced from its oxides with Coke (fuel) in a Blast furnace, producing Pig iron and carbon dioxide:

Yeast metabolizes Sugar to produce carbon dioxide and Ethanol, also known as alcohol, in the production of wines, beers and other spirits:

Glucose →

All aerobic organisms produce when they oxidize Carbohydrates, Fatty acids, and proteins in the mitochondria of cells. The large number of reactions involved are described easily. Refer to (aerobic, Anaerobic respiration and PH). PH (i.e. plants, Cyanobacteria) use another modus operandi: Plants absorb from the air, and, together with water, react it to form carbohydrates:

Carbon dioxide is Soluble in water, in which it spontaneously interconverts between and (Carbon). The relative concentrations of , and the deprotonated forms (Bicarbonate) and (Carbon) 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 productionEdit

Carbon dioxide is manufactured mainly from six processes:

  1. As a byproduct in NH3 and H plants, where methane is converted to ;
  2. From combustion of Wood and Fossil fuels;
  3. As a byproduct of Fermentation (biochemistry) of Sugar in the Brewing of Beer, Whisky and other Alcoholic Beverages;
  4. From thermal decomposition of limestone, , in the manufacture of Lime, ;
  5. As a byproduct of Sodium phosphate manufacture;
  6. Directly from natural carbon dioxide Spring (hydrosphere), where it is produced by the action of acidified water on Lime or Dolomite.


File:Soda bubbles macro.jpg
Carbon dioxide bubbles in a soft drink.

Carbon dioxide is used by the food industry, the oil industry, and the chemical industry. In medicine, up to 5% carbon dioxide is added to pure Oxygen for stimulation of breathing after Apnea and to stabilize the balance in blood.

A common type of industrial gas Laser is the Carbon.

Carbon dioxide can also be combined with Limonene oxide from orange peels or other Epoxides to create polymers and plastics.

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. In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points.

In the chemical industry, carbon dioxide is used for the production of Urea, Carbons and Bicarbonates, and Sodium salicylate.

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.

Liquid carbon dioxide (industry nomenclature R744 / R-744) was used as a refrigerant prior to the discovery of R-12 and is likely to enjoy a renaissance due to environmental concerns. 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 bars, systems require highly resistant components that have been already developed to serial production in many sectors. In car air conditioning, in more than 90% of all driving conditions, 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. Some applications: Coca-Cola has fielded -based beverage coolers and the US Army is interested in refrigeration and heating technology.

By the end of 2007, the global car industry is expected to decide on the next-generation refrigerant in car air conditioning. is one discussed option.(see The Cool War)

In the Earth's atmosphereEdit

File:Mauna Loa Carbon Dioxide.png
Atmospheric concentrations measured at Mauna Loa Observatory.

Carbon dioxide in Earth's atmosphere is considered a Trace gas currently occurring at an average concentration of about 385 parts per million by volume or 582 parts per million by mass. The mass of the Earth atmosphere is 5.14×1018 kg , so the total mass of atmospheric carbon dioxide is 3.0×1015 kg (3,000 gigatonnes). Its concentration varies seasonally (see graph at right) and also considerably on a regional basis: in urban areas it is generally higher and indoors it can reach 10 times the background atmospheric concentration.

Carbon dioxide is a Greenhouse gas; see Greenhouse effect for more.

Due to human activities such as the combustion of Fossil fuel and Deforestation, the concentration of atmospheric carbon dioxide has increased by about 35% since the beginning of the age of industrialization.

In 1999, 2,244,804,000 metric tons of were produced in the U.S. as a result of electric energy generation. This is an output rate of 0.6083 kg (1.341 pounds) per kWh.

1 kilogram of coal can generate 44/15 kilograms of CO2
15/44 kilograms of coal can generate 1 kilogram of CO2

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 maintained a slow decline until the age of industrialization, with a particularly swift reduction occurring 49 million years ago.

Up to 40% of the gas emitted by some Volcanoes during subaerial Volcanic eruptions is carbon dioxide. According to the best estimates, volcanoes release about 130-230 million tonnes (145-255 million tons) of 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 rise to above 75% overnight, sufficient to kill insects and small animals, but warm rapidly when sunlit and disperse by convection during the day Locally high concentrations of , produced by disturbance of deep lake water saturated with are thought to have caused 37 fatalities at Lake Monoun, Cameroon in 1984 and 1700 casualties at Lake Nyos, Cameroon in 1986. However, emissions of by human activities are currently more than 130 times greater than the quantity emitted by volcanoes, amounting to about 27 billion tonnes per year (30 billion tons).

In the oceansEdit

There is about 50 times as much carbon dissolved in the oceans in the form of and hydration products as exists in the atmosphere. The oceans act as an enormous Carbon, having "absorbed about one-third of all human-generated emissions to date." Generally, gas solubility decreases as water temperature increases. Accordingly carbon dioxide is released from ocean water into the atmosphere as ocean temperatures rise.

Most of the taken up by the ocean forms carbonic acid. Some is consumed in photosynthesis by organisms in the water, and a small proportion of that sinks and leaves the carbon cycle. There is considerable concern that as a result of increased in the atmosphere the acidity of seawater will increase and may adversely affect organisms living in the water. In particular, with increasing acidity, the availability of carbonates for forming shells decreases.

Biological roleEdit

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 aerobic. 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.

Role in photosynthesisEdit

Plants remove carbon dioxide from the atmosphere by photosynthesis, also called Carbon, which uses light energy to produce organic plant materials by combining carbon dioxide and water. Free oxygen is released as gas from the decomposition of water molecules, while the hydrogen is split into its protons and electrons and used to generate chemical energy via PH. This energy is required for the fixation of carbon dioxide in the Calvin cycle to form sugars. These sugars can then be used for growth within the plant through respiration.

Even when vented, carbon dioxide must be introduced into greenhouses to maintain plant growth, as the concentration of carbon dioxide can fall during daylight hours to as low as 200 ppm (a limit of C3 carbon fixation photosynthesis). Plants can potentially grow up to 50 percent faster in concentrations of 1,000 ppm when compared with ambient conditions.

Plants also emit during respiration, so it is only during growth stages that plants are net absorbers. For example a growing forest will absorb many tons of each year, however a mature forest will produce as much from respiration and decomposition of dead specimens (e.g. fallen branches) as used in Biosynthesis in growing plants. Regardless of this, mature forests are still valuable Carbons, helping maintain balance in the Earth's atmosphere. Additionally, and crucially to life on earth, phytoplankton photosynthesis absorbs dissolved in the upper ocean and thereby promotes the absorption of from the atmosphere.


Carbon emitting pollution in Taiwan.
Carbon dioxide content in fresh air varies between 0.03% (300 ppm) and 0.06% (600 ppm), depending on the location (see graphical map of in real-time).

A person's exhaled breath is approximately 4.5% carbon dioxide by volume.

Adaptation to increased levels of occurs in humans. Continuous inhalation of can be tolerated at three percent inspired concentrations for at least one month and four percent inspired concentrations for over a week. It was suggested that 2.0 percent inspired concentrations could be used for closed air spaces (ex. Submarine) since the adaptation is physiological and reversible. Decrement in performance or in normal physical activity does not happen at this level.

It is dangerous when inhaled in high concentrations (greater than 5% by volume, or 50,000 ppm). The current threshold limit value (TLV) or maximum level that is considered safe for healthy adults for an eight-hour work day is 0.5% (5,000 ppm). The maximum safe level for infants, children, the elderly and individuals with cardio-pulmonary health issues is significantly less.

These figures are valid for pure carbon dioxide. In indoor spaces occupied by people the carbon dioxide concentration will reach higher levels than in pure outdoor air. Concentrations higher than 1,000 ppm will cause discomfort in more than 20% of occupants, and the discomfort will increase with increasing concentration. The discomfort will be caused by various gases coming from human respiration and perspiration, and not by itself. At 2,000 ppm the majority of occupants will feel a significant degree of discomfort, and many will develop nausea and headaches. The concentration between 300 and 2,500 ppm is used as an indicator of indoor air quality.

Acute carbon dioxide toxicity is sometimes known as by the names given to it by miners: Blackdamp (also called choke damp or stythe). Miners would try to alert themselves to dangerous levels of carbon dioxide in a mine shaft by bringing a caged canary with them as they worked. The canary would inevitably die before reached levels toxic to people. Carbon dioxide caused a great loss of life at Lake Nyos in Cameroon in 1986, when an upwelling of -laden lake water quickly blanketed a large surrounding populated area. The heavier carbon dioxide forced out the life-sustaining oxygen near the surface, killing nearly two thousand people.

Carbon dioxide ppm levels (CDPL) are a surrogate for measuring indoor pollutants that may cause occupants to grow drowsy, get headaches, or function at lower activity levels. To eliminate most Indoor Air Quality complaints, total indoor CDPL must be reduced to below 600. NIOSH considers that indoor air concentrations that exceed 1,000 are a marker suggesting inadequate ventilation. ASHRAE recommends they not exceed 1,000 inside a space. OSHA limits concentrations in the workplace to 5,000 for prolonged periods. The U.S. National Institute for Occupational Safety and Health limits brief exposures (up to ten minutes) to 30,000 and considers CDPL exceeding 40,000 as "Immediately dangerous to life and health." People who breathe 50,000 for more than half an hour show signs of acute Hypercapnia, while breathing 70,000 – 100,000 can produce unconsciousness in only a few minutes. Accordingly, carbon dioxide, either as a gas or as dry ice, should be handled only in well-ventilated areas.

Human physiologyEdit

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% – 80%) is converted to Bicarbonate ions HCO3 by the enzyme Carbon in the red blood cells, by the reaction + H2O → H2CO3 → H+ + HCO3.

  • 5% – 10% is dissolved in the plasma

See alsoEdit


External linksEdit

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