Calcium carbonate is a chemical compound with the formula CaCO3. It is formed by three main elements: carbon, oxygen and calcium. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime, and is created when calcium ions in hard water react with carbonate ions creating limescale. It is commonly used medicinally 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) + 2 HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O (l)
*it releases carbon dioxide on 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 rocks, 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』