The subject of this film is the natural limestone cycle. Calcium carbonate is practically insoluble in pure water. But if the carbonated water flows over limestone the calcium carbonate reacts with the carbonic acid to form calcium hydrogen carbonate. That is however very soluble in water. Water in which lots of calcium hydrogen carbonate is dissolved is called hard water. If hard water evaporates or vaporizes, the mixture of dissolved carbon dioxide, carbonic acid and calcium hydrogen carbonate turns back into calcium carbonate and carbon dioxide. The carbon dioxide escapes as gas into the atmosphere, the insoluble calcium carbonate remains.