Precipitated CaCO3

Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which contains both of those minerals) and is the main component of pearls and the shells of marine organisms, snails, and eggs. Calcium carbonate is the active ingredient in agricultural lime and is created when calcium ions in hard water react with carbonate ions to create limescale. It is medicinally used as a calcium supplement or as an antacid, but excessive consumption can be hazardous.

Calcium carbonate shares the typical properties of other carbonates. Notably, it reacts with acids, releasing carbon dioxide:

CaCO3(s) + 2H+(aq) → Ca2+(aq) + CO2(g) + H2O (l)

It releases carbon dioxide upon heating, called a thermal decomposition reaction, or calcination, (to above 840 °C in the case of CaCO3), to form calcium oxide, commonly called quicklime, with reaction enthalpy 178 kJ / mole:

CaCO3 (s) → CaO (s) + CO2 (g)

Calcium carbonate will react with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate.

CaCO3 + CO2 + H2O → Ca(HCO3)2

This reaction is important in the erosion of carbonate rock, forming caverns, and leads to hard water in many regions.

An unusual form of calcium carbonate is the hexahydrate, ikaite, CaCO3, 6H2O. Ikaite is stable only below 6 °C.

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