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http://www.wisegeek.com/what-is-calcium-carbonate.htm

   

Calcium carbonate is an important chemical compound. It consists of one atom of calcium bonded to one atom of carbon and three atoms of oxygen. The molecular formula of calcium carbonate is CaCO₃. Common names for calcium carbonate are: limestone, calcite, aragonite, chalk, and marble. These are contain high amounts of calcium carbonate, but with different processes underlying their formation. Calcium carbonate is used in cements and mortars, producing lime, in the steel industry, glass industry, and as an ornamental stone.
The appearance of calcium carbonate is generally that of a white powder or stone. A characteristic quality of calcium carbonate is that it will fizz and release carbon dioxide upon contact with a strong acid, such as hydrochloric acid. After the carbon dioxide is released, the remainder is calcium oxide (CaO), commonly called quicklime.
When calcium carbonate comes into contact with water saturated with carbon dioxide, it forms a soluble compound, calcium bicarbonate. Underground, this often leads to the formation of caves. The reaction is as follows:
CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂
Calcium carbonate becomes marble when highly compressed and heated deep underneath the Earth’s surface. In caves, when temporarily dissolved by the above chemical mechanism, calcium carbonate creates magnificent speleothems:” cave formations such as stalagmites, stalagmites, curtains, and dozens of others.

calcium hydroxide  +  carbon dioxide    calcium carbonate  +  water.
  Ca(OH)_2(aq)        +        CO_2(g)                CaCO_3(s)     +         H₂O_(l)