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Carbon dioxide

Model of the carbon dioxide molecule. The carbon atom in black, the two oxygen atoms in red.Carbon dioxide (chemical formula CO2) is a colourless gas. It consists of a carbon atom covalently bonded to two oxygen atoms. It is about 60% denser than air. CO2 is soluble in water, in which it forms carbonic acid (H2CO3), a weak acid. At low temperatures (below -78.5°C, or -109.3°F) it turns directly into a solid, commonly called dry ice. This turns back into a gas, in a process called sublimation, at temperatures higher than -78.5°C. CO2 occurs naturally in the Earth's atmosphere in tiny quantities, but these are rising rapidly.

Hot springs in Yellowstone National Park, a natural source of carbon dioxide.


Natural sources of CO2 include volcanoes, geysers and hot springs. CO2 also occurs naturally dissolved in water. It is also present in oil and natural gas, and in carbonate rocks such as chalk and limestone (from which it is released when the rocks are dissolved in water). 

The human body produces approximately 1.0 kg (2.3 pounds) of carbon dioxide per day per person. This contains 290 g (0.63 pounds) of carbon.

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