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Concept
Calcium carbonate
Summary
Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skeletons and pearls. Materials containing much calcium carbonate or resembling it are described as calcareous. 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 has medical use as a calcium supplement or as an antacid, but excessive consumption can be hazardous and cause hypercalcemia and digestive issues.
Calcium carbonate shares the typical properties of other carbonates. Notably it
reacts with acids, releasing carbon dioxide (technically speaking, carbonic acid, but that disintegrates quickly to and ):
CaCO_3(s) {+} 2H^{+}(aq) -> Ca^{2+}(aq) + CO2(g) + H_2O(l)
releases carbon dioxide upon heating, called a thermal decomposition reaction, or calcination (to above 840 °C in the case of ), to form calcium oxide, CaO, commonly called quicklime, with reaction enthalpy 178 kJ/mol:
CaCO3(s)->[\Delta]CaO(s){+}CO2\uparrow
Calcium carbonate reacts with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate.
CaCO3(s){+}CO2(g){+}H2O(l)-> Ca(HCO3)2(aq)
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, . Ikaite is stable only below 8 °C.
The vast majority of calcium carbonate used in industry is extracted by mining or quarrying. Pure calcium carbonate (such as for food or pharmaceutical use), can be produced from a pure quarried source (usually marble).
Alternatively, calcium carbonate is prepared from calcium oxide.
Official source
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