Calcium bicarbonate (Ca(HCO₃)₂), also called calcium hydrogencarbonate, is a compound which exists only in solution. If a solution evaporates, the following reaction occurs: Ca(HCO₃)₂)(aq) → CO₂(g) + H₂O(l) + CaCO₃(s).

Calcium bicarbonate is formed when water containing carbon dioxide in solution (also known as carbonic acid) reacts with calcium carbonate.

This reaction is very important to the formation of stalactites, stalagmites, columns, and other speleothems within caves and, for that matter, in the formation of the caves themselves. Water containing atmospheric carbon dioxide passes through limestone, or other calcium carbonate containing minerals, converting part of the calcium carbonate to the bicarbonate, which is very water soluble. Later, in drier conditions, or upon a rise in temperature, the excess carbon dioxide is released from the solution of the bicarbonate, causing the much less soluble calcium carbonate to be deposited.

The reaction is very temperature sensitive, with a rise in temperature easily driving carbon dioxide out of the calcium bicarbonate solution.

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