Carbon dioxide (CO₂) is a chemical compound consisting of one carbon atom and two oxygen atoms. At ordinary temperatures and atmospheric pressure, it is a colorless, odorless, nonflammable gas. It occurs naturally in the Earth’s atmosphere and participates in biological and geological processes. It is also a greenhouse gas: its absorption of infrared radiation contributes to the planet’s thermal balance. Carbon dioxide has numerous industrial applications, while its increasing atmospheric abundance is a major driver of climate change. (pubchem.ncbi.nlm.nih.gov)
Molecular structure and physical properties
A carbon dioxide molecule has a linear structure, conventionally represented as O=C=O. Its central carbon atom forms two double covalent bonds, one to each oxygen atom. Its molar mass is approximately 44.01 grams per mole. The gas is about 1.5 times as dense as air under comparable conditions, a property relevant to its accumulation in poorly ventilated spaces. (pubchem.ncbi.nlm.nih.gov)
Solid carbon dioxide, known as dry ice, changes directly into gas at approximately −78.5 °C at one atmosphere of pressure. This process is called sublimation; no intervening liquid phase occurs under those conditions. Liquid carbon dioxide requires higher pressure. Its triple point, where solid, liquid, and gas coexist, is approximately −56.6 °C and 5.18 bar. (cdc.gov)
Above its critical temperature of approximately 31 °C and critical pressure of approximately 74 bar, carbon dioxide can exist as a supercritical fluid. In this state, it combines relatively high density with low viscosity. Its properties make it useful as a processing fluid and allow large quantities to occupy substantially less volume than gaseous carbon dioxide at atmospheric pressure. (netl.doe.gov)
Chemistry in water
Carbon dioxide dissolves in water and participates in a linked system of chemical equilibria. Some dissolved CO₂ reacts with water to form carbonic acid, H₂CO₃, which can dissociate into hydrogen and bicarbonate ions. Further dissociation produces carbonate ions. The relative abundance of these species depends strongly on pH. (oceanacidification.noaa.gov)
The initial reactions are commonly written:
CO₂ + H₂O ⇌ H₂CO₃
H₂CO₃ ⇌ H⁺ + HCO₃⁻
These equilibria are important in natural waters. When additional atmospheric carbon dioxide enters seawater, hydrogen-ion concentrations increase and carbonate availability decreases. This process, termed ocean acidification, lowers ocean pH and changes conditions for organisms that construct calcium carbonate shells or skeletons. “Acidification” describes a shift toward greater acidity; it does not require seawater to become acidic in the everyday sense of having a pH below seven. (oceanacidification.noaa.gov)
Biological and geological roles
Carbon dioxide is a central component of the carbon cycle, which transfers carbon among the atmosphere, organisms, soils, oceans, and rocks. During photosynthesis, plants and phytoplankton use light energy to incorporate carbon from CO₂ into organic compounds. This biological carbon fixation supplies the carbon used to build sugars and much of the material supporting food webs. (science.nasa.gov)
Carbon returns to the atmosphere through cellular respiration, decomposition, and combustion. Plants as well as animals respire, so vegetation can both absorb and release carbon dioxide. Seasonal differences between photosynthetic uptake and respiratory release contribute to the annual oscillation observed in atmospheric measurements, particularly in the Northern Hemisphere. (science.nasa.gov)
Geological processes operate over longer timescales. Carbon dioxide participates in rock weathering and ocean carbonate chemistry, while carbon can become incorporated into sediments and carbonate rocks. Volcanic activity returns some geological carbon to the atmosphere. These processes connect the relatively rapid biological cycle with carbon reservoirs that exchange material over millions of years. (science.nasa.gov)
Atmospheric abundance and climate
Before large-scale industrialization, atmospheric carbon dioxide concentrations were approximately 280 parts per million, or ppm. NOAA’s globally averaged marine-surface measurements gave an annual mean of approximately 422.8 ppm for 2024. Here, ppm expresses the number of CO₂ molecules per million molecules of dry air. A global annual average is distinct from measurements at an individual station or during a particular month. (gmd-prod.cmdl.noaa.gov)
The increase is primarily associated with burning fossil fuels, with additional contributions from land-use change. These activities transfer carbon into the atmosphere faster than natural sinks remove it. Carbon dioxide absorbs outgoing infrared radiation and contributes to the greenhouse effect. Increasing its concentration alters the exchange of energy between Earth and space. (climate.gov)
Industrial uses and carbon management
Commercial applications include beverage carbonation, food freezing, refrigeration, and fire extinguishing. Carbon dioxide also serves as a solvent or working fluid in industrial processes. Carbon dioxide utilization includes converting it into chemicals, fuels, biomass, or inorganic materials such as carbonate-based construction products. Physical applications leave the CO₂ molecule intact, whereas chemical conversion changes it into other substances. (pubchem.ncbi.nlm.nih.gov)
Carbon capture and storage separates carbon dioxide from industrial streams and places it in long-term storage, commonly underground. Direct air capture instead extracts CO₂ from ambient air. When atmospheric removal is coupled with durable storage, it constitutes carbon dioxide removal. Evaluating utilization pathways requires life-cycle assessment, because processing requirements and the eventual fate of product carbon affect their overall emissions balance. (energy.gov)
Exposure hazards
High concentrations of carbon dioxide can cause headache, dizziness, breathing difficulty, unconsciousness, and asphyxia. Liquid CO₂ and dry ice can cause frostbite. Although nonflammable, carbon dioxide is not chemically inert toward every substance: certain reactive metal powders can ignite or react explosively in it. These hazards are separate from the climatic effects of ordinary atmospheric concentrations. (cdc.gov)