Carbon Dioxide

Carbon Dioxide, $CO_2$

Preparation. Complete combustion of carbon and carbon-containing fuels in excess air gives $CO_2$:

In the laboratory it is conveniently made by treating calcium carbonate with dilute HCl:

Commercially it is obtained by heating limestone.

Properties. $CO_2$ is colourless, odourless, and only sparingly soluble in water — yet that small solubility gives it a huge biochemical/geochemical role. With water it forms carbonic acid, $H_2CO_3$, a weak dibasic acid that ionises in two steps (first to bicarbonate + $H_3O^+$, then bicarbonate to carbonate + $H_3O^+$); the $H_2CO_3$/$HCO_3^-$ buffer helps keep blood pH within its narrow range of 7.26–7.42.

$CO_2$ makes up about 0.03% of the atmosphere by volume and is removed by photosynthesis, in which green plants use light energy (via chlorophyll) to convert atmospheric $CO_2$ and water into glucose and oxygen:

Through photosynthesis, plants make food for themselves and, indirectly, for animals and humans.

Unlike CO, $CO_2$ is not poisonous, but rising combustion of fossil fuels plus $CO_2$ released during cement manufacture (from limestone decomposition) are increasing atmospheric $CO_2$ levels, feared to intensify the greenhouse effect and raise global temperature.

Uses. Solid $CO_2$ (dry ice), obtained by letting liquefied $CO_2$ expand rapidly, is used as a refrigerant for ice cream and frozen food. Gaseous $CO_2$ carbonates soft drinks and, being heavy and non-combustion-supporting, is used in fire extinguishers; large amounts are also used to manufacture urea. …