Calcium carbonate degrades into calcium oxide and calcium dioxide when it is heated high enough. If the heating process is done in water, the calcium oxide will react with the water and form calcium hydroxide. Calcium carbonate is often created when calcium ions and carbonate ions react in hard water.
Calcium carbonate is a common compound found in rocks such as limestone, chalk and marble. Its organic sources include snail shells, eggs and the shells of most sea life. The most important use of calcium carbonate is in architecture, where it is a ubiquitous and versatile building material. Calcium carbonate is also effective as an antacid and as a dietary calcium supplement.