Understanding Carbon Dioxide – CO2

Carbon Dioxide is a molecule with the formula CO2. It is formed from one carbon atom that is joined by double-bonds to two oxygen atoms, as below.

CO2 Molecule

The Three States of CO2


gaseous CO2 exists at temperatures of +31°C (87.8°F). It is colourless but has an odour.


when cooled to temperatures around +18°C (64.4°F) after being pressurised to approximately 69bar (1000.76 psi) CO2 becomes liquid.


releasing the pressure of liquid CO2 causes some of the liquid to change to a solid state with the remainder reverting to gas. The temperature of solid CO2 is -79°C (110.2°F).

Carbon dioxide has a density of 1.53g/cm3 at 21°C (69.8°F), which makes it heavier than air. Large volumes of CO2 sink to ground level and can cause asphyxiation.

Molecules are always in constant motion but at cold temperatures they move much slower than at warmer. The only time they stop is when the temperature is at absolute zero, which is 0° Kelvin (-273° C, -460° F). Cold molecules can be warmed up, however.

The Carbon Cycle

The Carbon Cycle describes how the chemical element carbon circulates through the Earth’s ecosystems. Below is a simplistic diagram illustrating this process.


Carbon Cycle

This process is very important for the existence of life on earth. Plants breathe in CO2 and exhale oxygen. Animals breathe in oxygen and exhale CO2. It is used in photosynthesis by plants to make sugars, used as a raw material for the production of polysaccharides, proteins, and other organic compounds. Herbivorous animals consume CO2 when eating vegetation and carnivorous animals, to a lesser extent, when they consume their prey. It is also produced when animals metabolize carbohydrates to produce energy.

Sources of CO2

Natural sources of CO2 include volcanoes, hot springs and geysers, rivers, lakes, glaciers, icecaps, seawater, and deposits of natural gas and petroleum.

CO2 is also produced from the combustion of carbon-based fuels, including natural gas (methane), coal, wood, and petroleum. It also occurs as a by-product in the manufacture of quicklime, used in making both steel and concrete, the industrial production of hydrogen, and the brewing industry.

Industrial uses include as a food additive, in making carbonated soft drinks, winemaking, as compressed gas for pneumatic systems, welding, in inflation canisters for lifejackets, for air and paintball guns, the drying of spices, the production of microelectromechanical systems, for the preparation of specimens for scanning with an electron microscope, removing caffeine from coffee beans, in fire extinguishers, to increase plant growth in greenhouses, in the oil industry, as a refrigerant, in lasers, to control the ph level of swimming pools, in the nuclear power industry, dry cleaning, and in entertainment as dry ice.

CO2 in the atmosphere

The Earth’s atmosphere is composed mostly of nitrogen (78%) and oxygen (21%). Trace gases make up most of the remaining 1%.

Composition of the atmosphere by gases

Gas Volume
Name Parts per million (ppm) Percentage
Nitrogen 780,840 78.084
Oxygen 209,460 20.946
Argon 9,340 0.9340
Carbon dioxide* 413.32 0.041332
Neon* 18.18 0.001818
Helium* 5.24 0.000524
Methane* 1.87 0.000187
Krypton* 1.14 0.000114
*Trace gases

CO2 is a trace gas because it equates to only 0.04% of the total atmosphere. It is counted as 413 ppm (as at time of writing), that is, of 1,000,000 parts of the atmosphere there are only 413 CO2 molecules to be found.

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