Carbon dioxide

Carbon dioxide
Ball-and-stick model of carbon dioxide
Space-filling model of carbon dioxide
Other names
  • Carbonic acid gas
  • Carbonic anhydride
  • Carbonic dioxide
  • Carbon(IV) oxide
  • R-744 (refrigerant)
  • R744 (refrigerant alternative spelling)
  • Dry ice (solid phase)
3D model (JSmol)
ECHA InfoCard 100.004.271
EC Number
  • 204-696-9
E number E290 (preservatives)
MeSH Carbon+dioxide
RTECS number
  • FF6400000
UN number 1013 (gas), 1845 (solid)
  • InChI=1S/CO2/c2-1-3 Y
  • InChI=1/CO2/c2-1-3
  • O=C=O
  • C(=O)=O
Molar mass 44.009 g·mol−1
Appearance Colorless gas
  • Low concentrations: none
  • High concentrations: sharp; acidic[1]
  • 1562 kg/m3 (solid at 1 atm (100 kPa) and −78.5 °C (−109.3 °F))
  • 1101 kg/m3 (liquid at saturation −37 °C (−35 °F))
  • 1.977 kg/m3 (gas at 1 atm (100 kPa) and 0 °C (32 °F))
Critical point (T, P) 304.128(15) K[2] (30.978(15) °C), 7.3773(30) MPa[2] (72.808(30) atm)
194.6855(30) K (−78.4645(30) °C) at 1 atm (0.101325 MPa)
1.45 g/L at 25 °C (77 °F), 100 kPa (0.99 atm)
Vapor pressure 5.7292(30) MPa, 56.54(30) atm (20 °C (293.15 K))
Acidity (pKa) 6.35, 10.33
−20.5·10−6 cm3/mol
Thermal conductivity 0.01662 W·m−1·K−1 (300 K (27 °C; 80 °F))[3]
  • 14.90 μPa·s at 25 °C (298 K)[4]
  • 70 μPa·s at −78.5 °C (194.7 K)
0 D
37.135 J/K·mol
214 J·mol−1·K−1
−393.5 kJ·mol−1
V03AN02 (WHO)
NFPA 704 (fire diamond)
Lethal dose or concentration (LD, LC):
90,000 ppm (human, 5 min)[7]
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 5000 ppm (9000 mg/m3)[8]
REL (Recommended)
TWA 5000 ppm (9000 mg/m3), ST 30,000 ppm (54,000 mg/m3)[8]
IDLH (Immediate danger)
40,000 ppm[8]
Safety data sheet (SDS) Sigma-Aldrich
Related compounds
Other anions
Other cations
Related carbon oxides
Related compounds
Supplementary data page
Carbon dioxide (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)

Carbon dioxide (chemical formula CO2) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transparent to visible light but absorbs infrared radiation, acting as a greenhouse gas. It is a trace gas in Earth's atmosphere at 417 ppm (about 0.04%) by volume, having risen from pre-industrial levels of 280 ppm.[9][10] Burning fossil fuels is the primary cause of these increased CO2 concentrations and also the primary cause of climate change.[11] Carbon dioxide is soluble in water and is found in groundwater, lakes, ice caps, and seawater. When carbon dioxide dissolves in water it forms carbonic acid (H2CO3), which causes ocean acidification as atmospheric CO2 levels increase.[12]

As the source of available carbon in the carbon cycle, atmospheric carbon dioxide is the primary carbon source for life on Earth. Its concentration in Earth's pre-industrial atmosphere since late in the Precambrian has been regulated by organisms and geological phenomena. Plants, algae and cyanobacteria use energy from sunlight to synthesize carbohydrates from carbon dioxide and water in a process called photosynthesis, which produces oxygen as a waste product.[13] In turn, oxygen is consumed and CO2 is released as waste by all aerobic organisms when they metabolize organic compounds to produce energy by respiration.[14] CO2 is released from organic materials when they decay or combust, such as in forest fires. Since plants require CO2 for photosynthesis, and humans and animals depend on plants for food, CO2 is necessary for the survival of life on earth.

Carbon dioxide is 53% more dense than dry air, but is long lived and thoroughly mixes in the atmosphere. About half of excess CO2 emissions to the atmosphere are absorbed by land and ocean carbon sinks.[15] These sinks can become saturated and are volatile, as decay and wildfires result in the CO2 being released back into the atmosphere.[16] CO2 is eventually sequestered (stored for the long term) in rocks and organic deposits like coal, petroleum and natural gas. Sequestered CO2 is released into the atmosphere through burning fossil fuels or naturally by volcanoes, hot springs, geysers, and when carbonate rocks dissolve in water or react with acids.

CO2 is a versatile industrial material, used, for example, as an inert gas in welding and fire extinguishers, as a pressurizing gas in air guns and oil recovery, and as a supercritical fluid solvent in decaffeination of coffee and supercritical drying.[17] It is also a feedstock for the synthesis of fuels and chemicals.[18][19][20][21] It is an unwanted byproduct in many large scale oxidation processes, for example, in the production of acrylic acid (over 5 million tons/year).[22][23][24] The frozen solid form of CO2, known as dry ice, is used as a refrigerant and as an abrasive in dry-ice blasting. It is a byproduct of fermentation of sugars in bread, beer and wine making, and is added to carbonated beverages like seltzer and beer for effervescence. It has a sharp and acidic odor and generates the taste of soda water in the mouth,[25] but at normally encountered concentrations it is odorless.[1]

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