hard water

Water in nature contains dissolved minerals. When it contains a lot of them, especially calcium and magnesium, we call it hard water. Calcium and magnesium get into water underground, where limestone reacts with carbon dioxide to form calcium bicarbonate, a salt that dissolves in water. Such water usually tastes very good, but it is hard on household appliances. Whenever it is heated, the dissolved calcium bicarbonate breaks down back into carbon dioxide and insoluble limestone (calcium carbonate). This settles on sanitary fittings and household appliances and damages their heating elements. In tea it forms a thin film on the surface, and coffee tastes worse.