Carbon Dioxide: Properties, Biological Roles, and Industrial Applications

Carbon dioxide (CO2), also known as carbonic anhydride or carbon(IV) oxide, is a colorless gas essential to life on Earth. While often discussed in the context of climate change, it is a versatile molecule with critical roles in biological respiration, plant growth, and a vast array of industrial processes. From the solid form known as dry ice to its use as a supercritical solvent, carbon dioxide exhibits a wide range of physical and chemical behaviors.

At a molecular level, carbon dioxide consists of one carbon atom double-bonded to two oxygen atoms in a linear arrangement. This structure results in a molecule with no dipole moment, meaning it is non-polar despite the polarity of the individual C=O bonds.

Structural formula of carbon dioxide with bond length

Key Facts

Chemical Formula: CO2 (Molar mass: 44.009 g·mol).
Physical States: Exists as a gas at standard temperature, a liquid under pressure, and a solid (dry ice) at -78.5 °C.
Biological Role: A byproduct of cellular respiration and a primary reactant in photosynthesis.
Environmental Impact: A significant greenhouse gas; atmospheric levels have risen from pre-industrial levels of 280 ppm to 421 ppm as of May 2022.
Industrial Use: Major applications include urea production for fertilizer and enhanced oil recovery.

Chemical and Physical Properties

Electrical conductivity of carbondioxide saturated desalinated water when heated from 20 to 98 °C. The shadowed regions indicate the error bars associated with the measurements. A comparison with the temperature dependence of vented desalinated water can be found here .

Carbon dioxide is characterized by its stability and specific phase transitions. Unlike water, it does not exist as a liquid at standard atmospheric pressure; instead, it undergoes sublimation, transitioning directly from a solid to a gas at -78.46 °C.

When dissolved in water, carbon dioxide reacts to form carbonic acid (H2CO3), a weak acid that dissociates into bicarbonate (HCO3-) and carbonate (CO32-) ions. This equilibrium is fundamental to the pH regulation of the oceans and human blood.

Phase Behavior and Density

The behavior of CO2 varies significantly with temperature and pressure. At its critical point (30.978 °C and 7.3773 MPa), it becomes a supercritical fluid, possessing the properties of both a gas and a liquid, which makes it an excellent industrial solvent.

Pressure–temperature phase diagram of carbon dioxide. Note that it is a log-lin chart.

The solid phase, dry ice, is used extensively for cooling and preservation due to its extremely low temperature and the fact that it leaves no liquid residue upon evaporation.

Pellets of "dry ice", a common form of solid carbon dioxide

Molecular Vibrations

The CO2 molecule can undergo several types of oscillations, including symmetric stretching, antisymmetric stretching, and bending modes. These vibrations allow the molecule to absorb and re-emit infrared radiation, which is the mechanism behind its role as a greenhouse gas.

Stretching and bending oscillations of the CO2 molecule. Upper left: symmetric stretching. Upper right: antisymmetric stretching. Lower line: degenerate pair of bending modes.

Biological and Environmental Role

Rising levels of CO2 threatened the Apollo 13 astronauts, who had to adapt cartridges from the command module to supply the carbon dioxide scrubber in the Apollo Lunar Module, which they used as a lifeboat.

Carbon dioxide is a central component of the Earth's carbon cycle. In plants, it is captured through carbon fixation during the Calvin cycle, where it is converted into organic compounds using energy from sunlight—a process known as photosynthesis.

Overview of the Calvin cycle and carbon fixation

Conversely, animals and plants release CO2 through respiration, where organic compounds are broken down to produce energy, water, and carbon dioxide.

Overview of photosynthesis and respiration. Carbon dioxide (at right), together with water, form oxygen and organic compounds (at left) by photosynthesis (green), which can be respired (red) to water and CO2.

Human Physiology and Toxicity

In humans, CO2 is transported in the blood primarily as bicarbonate ions, with smaller portions dissolved in plasma or bound to hemoglobin. The concentration of CO2 in the blood is a primary trigger for the regulation of breathing.

While necessary for life, high concentrations of CO2 are toxic. Levels above 10,000 ppm can lead to cognitive impairment, while concentrations between 20,000 and 50,000 ppm can cause headaches, sleepiness, and nausea.

Symptoms of carbon dioxide toxicity, by increasing volume percent in air[45]

Atmospheric and Oceanic Impact

The Keeling Curve illustrates the steady rise of atmospheric CO2 since 1958, driven largely by anthropogenic sources such as fossil fuel combustion and deforestation.

Atmospheric CO2 concentration measured at Mauna Loa Observatory in Hawaii from 1958 to 2023 (also called the Keeling Curve). The rise in CO2 over that time period is clearly visible. The concentration is expressed as μmole per mole, or ppm.

As the atmosphere's CO2 levels increase, more of the gas is absorbed by the oceans. This leads to ocean acidification, which lowers the pH of seawater and makes it difficult for marine organisms, such as pteropods, to maintain their calcium carbonate shells.

Pterapod shell dissolved in seawater adjusted to an ocean chemistry projected for the year 2100

Industrial and Commercial Applications

A carbon dioxide sensor that measures CO2 concentration using a nondispersive infrared sensor

Carbon dioxide is utilized across numerous industries due to its chemical properties and inert nature.

Common Commercial Uses of Carbon Dioxide
Application	Primary Use Case	Key Characteristic
Agriculture	Urea production for fertilizer	Chemical precursor
Energy	Enhanced Oil Recovery (EOR)	Supercritical fluid properties
Food & Beverage	Carbonation in soft drinks	Solubility in liquids
Safety	Fire extinguishers	Displaces oxygen (inert)
Cooling	Dry ice for food preservation	Low sublimation point
Technology	CO2 Lasers	Specific vibrational energy

In the food industry, CO2 is used to carbonate beverages and as a preservative (E290). In safety applications, it is used in fire extinguishers to displace oxygen and smother fires.

More specialized uses include the use of CO2 as a refrigerant (R-744) and in the operation of high-power carbon-dioxide lasers.

Frequently Asked Questions

Annual CO2 flows from anthropogenic sources (left) into Earth's atmosphere, land, and ocean sinks (right) since the 1960s. Units in equivalent gigatonnes carbon per year.[89]

Pie chart of commercial CO2 use. See caption for description. — The biggest commercial uses of CO2 are in producing urea for fertilizer and in extracting oil from the ground. Beverages, food, metal fabrication, and other uses account for 3%, 3%, 2%, and 4% of commercial CO2 use, respectively.[106]

Dry ice used to preserve grapes after harvest

Comparison of the pressure–temperature phase diagrams of carbon dioxide (red) and water (blue) as a log-lin chart with phase transitions points at 1 atmosphere

What is the difference between carbon dioxide and carbon monoxide?

Carbon dioxide (CO2) consists of one carbon atom and two oxygen atoms and is a natural byproduct of respiration. Carbon monoxide (CO) consists of one carbon atom and one oxygen atom; it is a highly toxic gas typically produced by incomplete combustion.

Why is carbon dioxide called a greenhouse gas?

Carbon dioxide is a greenhouse gas because its molecular structure allows it to absorb and re-emit infrared radiation (heat) escaping from the Earth's surface, trapping warmth in the atmosphere.

What happens when carbon dioxide dissolves in water?

When CO2 dissolves in water, it reacts to form carbonic acid (H2CO3), which then dissociates into hydrogen ions, bicarbonate, and carbonate ions, increasing the acidity of the solution.

What is dry ice and why is it called that?

Dry ice is the solid form of carbon dioxide. It is called "dry" because it sublimes—meaning it turns directly from a solid into a gas without passing through a liquid phase at standard pressure.

At what concentration does carbon dioxide become dangerous to humans?

While normal indoor levels are around 1,121 ppm, cognitive impairment can begin at 10,000 ppm. Drowsiness occurs between 10,000 and 20,000 ppm, and severe symptoms like nausea and loss of attention appear between 20,000 and 50,000 ppm.