Carbon dioxide

Carbon dioxide, or CO₂, is an atmospheric gas composed of one carbon and two oxygen atoms. In solid form it is sometimes known as dry ice, since it sublimes at atmospheric pressure. As a gas it is opaque to infrared energy, and odorless and colorless.

It is a colorless, heavy, irrespirable gas, extinguishing flame, and when breathed destroys life. It can be reduced to a liquid and solid form by intense pressure. CO₂ produced in the fermentation of liquors, and by the combustion and decomposition of organic substances, or other substances containing carbon. It is formed as a product of combustion and respiration. Water will absorb its own volume of it, and more than this under pressure, and in this state becomes soda water also known as carbonated water of soda pop and natural springs.

CO₂ is a minor but very important component of Earth's atmosphere (because it traps infrared radiation). Atmospheric CO₂ has increased about 25 percent since the early 1800s, with an estimated increase of 10 percent since 1958 (burning fossil fuels such as coal and petroleum is the leading cause of increased CO₂, deforestation the second major cause). The increased amounts of CO₂ in the atmosphere enhances the greenhouse effect, blocking heat from escaping into space and contributing to the global warming of Earth's lower atmosphere.

In biochemistry it is commonly produced by animals that break down sugars with oxygen as part of their metabolism. Carbon dioxide is removed from the atmosphere by plants, that use photosynthesis of carbon dioxide and water to produce free oxygen gas and hydrocarbon plant materials.