Carbon dioxide (chemical formula: CO2) is a chemical compound composed of two oxygen atoms covalently bonded to a
single carbon atom. It is a gas at standard temperature and pressure. Carbon dioxide exists in Earth's atmosphere
currently at a globally averaged concentration of approximately 385 parts per million by volume. Carbon dioxide is a
greenhouse gas as it transmits visible light but absorbs strongly in the infrared and near-infrared.
Carbon dioxide is used by plants during photosynthesis to make sugars which may either be consumed again in
respiration or used as the raw material to produce polysaccharides such as starch and cellulose, proteins and the wide
variety of other organic compounds required for plant growth and development. It is produced during respiration by plants,
and by all animals, fungi and microorganisms that depend on living and decaying plants for food, either directly or indirectly.
It is, therefore, a major component of the carbon cycle. Carbon dioxide is generated as a by-product of the combustion of
fossil fuels or the burning of vegetable matter, among other chemical processes. Over very long time scales (thousands to
millions of years), concentrations are influenced by emissions from volcanoes and other geothermal processes such as
hot springs and geysers and by the dissolution of carbonates in crustal rocks.
Carbon dioxide has no liquid state at pressures below 5.1 atm. At 1 atm it is a solid at temperatures below −78 °C. In its
solid state, carbon dioxide is commonly called dry ice.
CO2 is an acidic oxide: an aqueous solution turns litmus from blue to pink.
CO2 in concentrations of 7% to 10% cause dizziness, headache, visual and hearing dysfunction, and unconsciousness
within a few minutes to an hour
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