Calcium bicarbonate is a chemical with the formula C2H2CaO6, and it is one of the major causes of hard water. It is one of the salts of carbonic acid, containing the negative ion HCO3. Calcium bicarbonate typically exists solely in a water solution.
All water molecules that have contact with the atmosphere absorb some carbon dioxide. Some water, such as the water found in streams, also encounter s rocks and other sediments. The water molecules of this water gain metal ions such as magnesium and calcium, which can react with the carbon dioxide and water. As a result, compounds such as calcium bicarbonate form and create hard water.