Topic summary
Carbon dioxide
- Carbonic acid gas
- Carbonic anhydride
- Carbonic dioxide
- Carbonic oxide
- Carbon(IV) oxide
- Methanedione
- R-744 (refrigerant)
- R744 (refrigerant alternative spelling)
- Dry ice (solid phase)
- 204-696-9
- FF6400000
- InChI=1S/CO2/c2-1-3Key: CURLTUGMZLYLDI-UHFFFAOYSA-N
- InChI=1/CO2/c2-1-3Key: CURLTUGMZLYLDI-UHFFFAOYAO
- O=C=O
- C(=O)=O
- Low concentrations: none
- High concentrations: sharp; acidic
- 1562kg/m (solid at 1 atm (100 kPa) and −78.5 °C (−109.3 °F))
- 1101kg/m (liquid at saturation −37 °C (−35 °F))
- 1.977kg/m (gas at 1 atm (100 kPa) and 0 °C (32 °F))
pKa1 = 3.6
pKa1(apparent) = 6.35
pKa2 = 10.33
- 14.90 μPa·s at 25 °C (298 K)
- 70μPa·s at −78.5 °C (194.7 K)
entropy(S298)
formation(ΔfH298)
verify ( ?)
Carbon dioxide is a chemical compound with the chemical formulaCO2. It is made up of molecules that each have one carbon atom covalentlydouble bonded to two oxygen atoms. It is found in a gas state at room temperature and at normally-encountered concentrations it is odorless. As the source of carbon in the carbon cycle, atmospheric CO2 is the primary carbon source for life on Earth. In the air, carbon dioxide is transparent to visible light but absorbs infrared radiation, acting as a greenhouse gas. Carbon dioxide is soluble in water and is found in groundwater, lakes, ice caps, and seawater.
It is a trace gasin Earth's atmosphere at 428 parts per million (ppm), or about 0.043% (as of July 2025) having risen from pre-industrial levels of 280 ppm or about 0.028%. Burning fossil fuels is the main cause of these increased CO2 concentrations, which are the primary cause of climate change.
Its concentration in Earth's pre-industrial atmosphere since late in the Precambrian was regulated by organisms and geological features. 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. 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. CO2 is released from organic materials when they decay or combust, such as in forest fires. When carbon dioxide dissolves in water, it forms carbonate and mainly bicarbonate (HCO3), which causes ocean acidification as atmospheric CO2 levels increase.
Carbon dioxide is 53% denser 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. These sinks can become saturated and are volatile, as decay and wildfires result in the CO2 being released back into the atmosphere. CO2, or the carbon it holds, is eventually sequestered (stored for the long term) in rocks and organic deposits like coal, petroleum and natural gas.
Nearly all CO2 produced by humans goes into the atmosphere. Less than 1% of CO2 produced annually is put to commercial use, mostly in the fertilizer industry and in the oil and gas industry for enhanced oil recovery. Other commercial applications include food and beverage production, metal fabrication, cooling, fire suppression and stimulating plant growth in greenhouses.