What does liquid carbon dioxide look like

Structural formula
Surname carbon dioxide
other names

CO2, carbon dioxide, carbon dioxide, carbon (IV) oxide

Molecular formula CO2
CAS number 124-38-9
Brief description colorless, odorless gas
Molar mass 44.0099 g / mol
Physical state gaseous
density 1.9767 kg m–3 (0 ° C, 1013 mbar)[1]
Melting point -56.6 ° C (5.3 bar)[1]
boiling point -78.5 ° C (sublimation)[1]
Vapor pressure

57.258 bar[1] (20 ° C)


good in water[1]

safety instructions

9100 mg m–3[1]

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions.

carbon dioxide (in normal parlance mostly Carbon dioxide, but often incorrectly carbonic acid called) is a chemical compound of carbon and oxygen and belongs to the group of carbon oxides in addition to carbon monoxide (also carbon monoxide), carbon suboxide and the unstable carbon trioxide.

Carbon dioxide is a colorless and odorless gas. With a concentration of approx. 0.04% (currently 381 ppm corresponding to 0.0381%) it is a natural component of the air. It arises both from the complete combustion of carbon-containing substances under sufficient oxygen and in the organism of living beings as a by-product of cellular respiration. The CO2 is released through the breath. Conversely, plants, some bacteria and archaea are able to CO2 converting it into biomass through carbon dioxide fixation. For example, plants produce from inorganic CO during photosynthesis2Glucose.


Waste product from the energy industry

Carbon dioxide is produced by the combustion of carbonaceous fuels, including all fossil fuels. For a given energy source, the amount of CO produced is2 directly on the amount of fuel and thus the converted energy. Modern systems and operating processes can make better use of the energy contained in the fuel than they used to, but they cannot prevent the formation of the gas.

This production is around 36 billion tons a year worldwide. In the absence of an effective and economical process for capturing carbon dioxide, this amount escapes into the atmosphere and contributes to global warming (see below).


Technically, carbon dioxide is obtained by burning coke with excess air or as a by-product of lime burning (~ 530 million tons per year) and subsequent cleaning (e.g. binding to potassium carbonate to hydrogen carbonate and subsequent release by heating).

Natural gas sources (mineral water) are also used for extraction.

On a laboratory scale

In the laboratory, carbon dioxide is produced by releasing carbonates from acids.


A simple detection of carbon dioxide is possible with an aqueous calcium hydroxide solution (lime water). For this purpose, the gas to be examined is introduced into the solution. If the gas contains CO2, then calcium carbonate (lime) precipitates as a whitish solid and the solution becomes cloudy. (please referLime water sample)

Carbon dioxide reacts with calcium hydroxide to form water and calcium carbonate.

Physical Properties

The carbon dioxide molecule has a linear structure. Although the carbon-oxygen bonds are polar, their electrical dipole moments cancel each other out due to the molecular symmetry, so that the molecule itself does not have an electrical dipole moment. Nevertheless, due to its internal dipole moments, carbon dioxide is readily soluble in water and absorbs a few narrow parts of the electromagnetic spectrum in the infrared radiation range.

Carbon dioxide is used in technology in the solid state under the name dry ice. It does not melt, but sublimes at −78 ° C. However, below the critical temperature of 31 ° C, it can be compressed to a colorless liquid by increasing the pressure. At room temperature, a pressure of approx. 60 bar is necessary for this, the critical pressure at the critical temperature is approx. 73.7 bar[1]. In liquid form, carbon dioxide is traded in pressure bottles.

Chemical properties

Carbon dioxide dissolved in water forms carbonic acid, H2CO3, but more than 99% of the carbon dioxide is only physically dissolved. The carbonic acid as such is in equilibrium with its dissociation products (species) hydrogen carbonate ("bicarbonate", HCO)3) and carbonate (CO32–), which are in a proportion to each other depending on the pH value. If the oxonium ions formed during dissociation (H.+, actually H3O+) by adding a lye with hydroxide ions (OH) from, the quantitative ratio shifts in favor of carbonate.


During the sublimation of dry ice, a white mist is created from the cold CO2-Air mixture and condensing humidity, which was previously used as an effect in stage technology. Today, for example, there are fog cooling attachments for normal evaporator fog machines that operate with liquid CO2 operate.

Many drinks contain carbon dioxide in order to have a better refreshment effect when drinking. With some drinks it is produced through fermentation (beer, sparkling wine), with others it is added artificially (lemonade, soda water) or natural mineral water containing carbon dioxide is used. As a food additive, it bears the designation E 290. During production, carbon dioxide is pumped into the drink under high pressure, about 0.2% of which reacts with water to form carbonic acid; most of it is dissolved in the water as a gas. If the pressure drops when the vessel is opened, nucleation occurs, so that the gas, which is now excessively dissolved, emerges in the form of bubbles and rises. The bubble formation of the gas and the sour taste of the carbon dioxide on the tongue when drinking stimulate the taste sensory cells, which has a refreshing effect.

Carbon dioxide is also used in fire extinguishers because it displaces oxygen from the source of the fire (see also CO2 extinguishers, fire fighting, extinguishing agents).

Carbon dioxide is used as a fertilizer in greenhouses. The reason is the CO produced by photosynthetic consumption2-Lack of fresh air when there is insufficient supply, especially in winter when the ventilation is closed, because plants CO2 need as a basic substance. The carbon dioxide is either introduced directly as a pure gas (relatively expensive) or as a combustion product from propane or natural gas (coupling of fertilization and heating). The possible increase in yield depends on how severe the lack of CO2 and how strong the light is for the plants. Carbon dioxide is also used in aquaristics as a fertilizer for aquatic plants (CO2Diffuser). The CO can also be reduced by adding organic matter2-Content in the water are increased (breathing, but at the expense of the oxygen content). (See also:Carbon dioxide fertilization)

Supercritical carbon dioxide has a high solubility for non-polar substances and can replace toxic organic solvents. It is used as an extraction agent, for example for the extraction of natural substances such as caffeine (production of decaffeinated Coffee by decaffeination), and used as a solvent for cleaning and degreasing, for example of wafers in the semiconductor industry and recently also of textiles (dry cleaning). There is currently intensive research into using supercritical carbon dioxide as a reaction medium for the production of fine chemicals (e.g. for the production of flavorings), as isolated enzymes often remain active in them and no solvent residues (in contrast to organic solvents) remain in the products.

Carbon dioxide is used as a refrigerant under the designation R744 or R-744 in vehicle and stationary air conditioning systems, in industrial refrigeration, supermarket and transport refrigeration as well as in vending machines. It has a large volumetric cooling capacity (higher efficiency in a given volume), high environmental compatibility (global warming potential compared to refrigerants used today about 1/1000 per kg; no ozone depletion potential; extraction from industrial waste gas) and can at the same time be used in heat cycles such as in Hot water pumps and vehicle heaters can be used.[2]

Carbon dioxide is also used as a protective gas in welding technology - either in its pure form or more often as an additive to argon and / or helium. Since carbon dioxide is thermodynamically unstable at high temperatures, it is not referred to as an inert gas, but as an active gas.

CO2 is also used in laxatives (suppositories). A chemical reaction during the dissolution of the suppository creates CO2 released and stretches the intestines, which in turn triggers the stool reflex.

Increasingly, CO2 used in conjunction with an automatable blasting process to produce high-purity surfaces. With its combination of mechanical, thermal and chemical properties, CO2-Snow dissolve and remove various types of surface contamination without leaving any residue.

In so-called CO2Recently, pigs have also been stunned before slaughter in plants. To do this, they are lowered in groups in a paternoster elevator into a pit that has at least 90% CO2 contains and lose consciousness in the process.

CO2 in the atmosphere and greenhouse effect


Main article: greenhouse effect

Carbon dioxide absorbs part of the thermal radiation (infrared radiation), while shorter-wave radiation, i.e. H. most of the solar radiation, can happen. This property makes carbon dioxide a so-called greenhouse gas. After water vapor, carbon dioxide is the most effective of the greenhouse gases in terms of its proportion, although the specific effectiveness of methane and ozone are higher. All greenhouse gases together increase the mean temperature on the earth's surface from approx. −18 ° C to +15 ° C (natural greenhouse effect). Carbon dioxide has a share of approx. 9 to 26% of this overall effect and is therefore largely responsible for the earth-friendly climate.

The CO2-The proportion in the earth's atmosphere has been subject to considerable fluctuations over the course of the earth's history, which have various biological, chemical and physical causes. For at least 650,000 years, however, the proportion has always been below 280 ppm.[3] The CO2-Concentration over the past 10,000 years has remained relatively constant at 280 ppm. The balance of the carbon dioxide cycle was thus largely balanced during this time. With the beginning of industrialization in the 19th century, the CO increased2-Content in the atmosphere to so far 381 ppm (2006) and increases z. Currently by an average of 1.5 to 2 ppm per year.[4]

This increase is due to the anthropogenic, i.e. H. man-made, CO2-Emissions of approx. 36.3 Gt or approx. 9.9 Gt of carbon annually (8.4 GtC from burning fossil raw materials and 1.5 ± 0.5 GtC from land use)[4] causes. This makes up only a small proportion of the carbon dioxide, which comes mainly from natural sources, of around 550 Gt CO annually2 or 150 Gt carbon[5], however, causes a net inflow, since the balance of natural CO2-Circuit is zero. The anthropogenic CO2-Emissions are partly absorbed by the natural carbon dioxide sinks, so that only about 45% of anthropogenic carbon dioxide accumulates in the atmosphere.

The vast majority of scientists believe that a man-made increase in greenhouse gases in the atmosphere contributes to the anthropogenic greenhouse effect that leads to global warming. CO makes a significant contribution to warming2, which is released through the combustion of the fossil fuels crude oil, natural gas and coal, while the combustion of biomass and the fuels obtained from it only account for CO2- Releases quantities that had previously been photosynthetically bound. The consequences of global warming should be reduced through climate protection.

Since the 1990s at the latest, it has generally been recognized in science that there is a statistically significant climate change and that one of the causes is the increase in the concentration of carbon dioxide in the atmosphere. This suspicion, which was initially associated with greater uncertainties, has been confirmed more and more in the course of research and after fierce controversy about global warming and is now largely a scientific consensus.[6] The observed temperature data cannot be explained without taking greenhouse gases into account.[7]

Physiological effects and hazards

CO2-Concentrations (vol-%) in air and effects on humans:

  • 0.038%: Current concentration in the air
  • 0,15 %: Hygienic indoor air guide value for fresh air
  • 0.3%: MIK value below which there are no health concerns with long-term exposure
  • 0.5% (9 g / m³): MAK limit value for daily exposure of eight hours per day
  • 1.5%: increase in respiratory time volume by more than 40%.
  • 4%: breathing air when exhaling
  • 5%: occurrence of headache, dizziness and loss of consciousness
  • 8%: unconsciousness, death occurs after 30–60 minutes

There are always accidents with CO2. In wine cellars, feed silos, wells and cesspools, fermentation processes can cause considerable amounts of CO2 form. The fermentation of one liter of must produces up to 50 liters of fermentation gas. If insufficient ventilation is provided, dangerous concentrations will be formed due to the higher density of CO2 compared to air, especially near the ground (Carbon dioxide lake).

The direct harmful effect on animals and humans can in individual cases be based on the displacement of oxygen in the air. The widely held view, CO2 is in itself harmless and only works by displacing the vital oxygen, but it is wrong. Hence the old one Candle rehearsal not useful for recognizing dangerous oxygen shortages. By displacing the air (lowering of the O2Partial pressure to less than 130 mbar) due to the heavier carbon dioxide it can additionally on the harmful effects of CO2 also suffocate from lack of oxygen.

CO dissolved in the blood2 Activates the breathing center of the brain in a physiological (natural) and slightly increased concentration, but in a significantly higher concentration it leads to a reduction or even removal of the reflex breathing stimulus (respiratory depression, respiratory arrest). These effects occur much more quickly than suffocation.

From around 5 percent CO2 In the inhaled air headaches and dizziness occur, with higher concentrations accelerated heartbeat (tachycardia), rise in blood pressure, shortness of breath and unconsciousness (the so-called CO2-Anesthesia). CO2-Concentrations of 8 percent and more lead to death within 30 to 60 minutes.

In addition, carbon dioxide has an indirect effect on the oxygen balance in the blood. If there is more carbon dioxide in the air or in the fresh water, the pH value in the blood is reduced via the dissociation equilibrium of the carbonic acid - the blood becomes "more acidic". Hemoglobin is affected by this drop in pH. At a lower pH value, its O decreases2-Binding capacity. That means with the same O2-Air content, less oxygen can be bound and transported by hemoglobin. This fact is described by the Bohr effect and the Haldane effect. The concentration of CO is in the tissue where the oxygen is to be given off2 higher (= lower pH value, lower O2-Binding capacity) and thus facilitates the O2-Delivery. In the lungs, the situation is reversed and thus promotes the "loading" of the hemoglobin with oxygen.

This indirect effect via the pH value of the blood must be differentiated from the far greater toxicity of carbon monoxide. As a complexing agent, carbon monoxide irreversibly masks the iron core of hemoglobin and thus prevents the binding of oxygen in the red blood cells. This is a different (more effective) molecular mechanism than that of carbon dioxide.

Time and again, entire families fall victim to fermentation gas poisoning because several people inhale carbon dioxide themselves while rescuing a family member and become unconscious. The first aider is only putting himself in danger with an attempt to rescue himself - nobody can carry an unconscious person out of a cellar with bated breath. Instead, a ventilation system (if available) must be switched on and an emergency call made.

Rescuing a casualty from CO2Suspicious situations (wine cellar, etc.) is only possible by professional emergency services (fire brigade) with self-contained breathing apparatus.

In rare cases, natural disasters with carbon dioxide also occur; the best known occurred in 1986 at Lake Nyos in Cameroon.

See also


  • Eike Roth: Global environmental problems - causes and possible solutions. Friedmann, Munich 2004. (Greenhouse effect, including its causation and discussion of the anthropogenic influence.) ISBN 3-933431-31-X
  • Pörtner: Effects of CO2-Entry and temperature increase in the marine biosphere (pdf 1.3 MB, 85 p.)
  • Bauer, Kurt: On the importance of carbonic acid in carp ponds. Österreichs Fischerei 44/1991 pp 49-64


  1. abcdefGHi Entry to carbon dioxide in the GESTIS substance database of the BGIA, accessed on August 31, 2007 (JavaScript required)
  2. The natural refrigerant R744 (CO2)
  3. Siegenthaler, Urs, Thomas F. Stocker, Eric Monnin, Dieter Lüthi, Jakob Schwander, Bernhard Stauffer, Dominique Raynaud, Jean-Marc Barnola, Hubertus Fischer, Valérie Masson-Delmotte and Jean Jouzel (2005): Stable Carbon Cycle – Climate Relationship During the Late Pleistocene, in: Science, Vol. 310, No. 5752, pp. 1313-1317, November 25, see abstract online
  4. ab Josep Canadella, Corinne Le Quéré, Michael Raupacha, Christopher Fielde, Erik Buitenhuisc, Philippe Ciaisf, Thomas Conwayg, Nathan Gillettc, R. Houghtonh and Gregg Marland (2007): Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks, in: Proceedings of the National Academy of Sciences, online (PDF)
  5. Intergovernmental Panel on Climate Change (2001): Climate Change 2001 - IPCC Third Assessment ReportCO2-Circle
  6. Oreskes, Naomi (2004): The Scientific Consensus on Climate Change, in: Science Vol. 306 of December 4th (PDF)
  7. Meehl, Gerald A., Warren M. Washington, Caspar M Ammann, Julie M. Arblaster, T. M. L. Wigleiy and Claudia Tebaldi (2004): Combinations of Natural and Anthropogenic Forcings in Twentieth-Century Climate, in: Journal of Climate, Vol. 17, October 1, pp. 3721–3727 (PDF)

Categories: Oxygen Compound | Carbon compound | gas