Which compound is the principal component of limestone and has the formula CaCO₃?

Calcium carbonate is the principal component of limestone and has the formula CaCO₃.

Calcium carbonate is an ionic compound containing calcium ions and carbonate ions. It occurs naturally in limestone, chalk, and marble, although these rocks differ in texture and geological history. Many shells and coral skeletons also contain calcium carbonate, commonly in the mineral forms calcite or aragonite.

The compound reacts with acids to release carbon dioxide gas. This reaction explains why acidic rainwater can gradually dissolve limestone and why dilute acid fizzes on chalk or marble. Heating calcium carbonate strongly produces calcium oxide, called quicklime, and carbon dioxide; this reaction is central to cement and lime production.

Calcium carbonate is sometimes confused with calcium bicarbonate. Calcium bicarbonate is found only in aqueous solution under suitable conditions and is associated with temporary water hardness, whereas solid calcium carbonate forms rocks, shells, and mineral deposits. The same compound is also used in construction materials, paper, paints, and some medicines.