Water hardness is determined by how much of the minerals calcium and magnesium it contains. Total hardness is composed of permanent and carbonate hardness ("lime"). When water with carbonate hardness is heated, its high lime content can lead to limescale deposits. This scale leads to increased energy consumption and even causes some appliances to stop working. Hard water means more detergent is needed for washing and that frothing is more difficult. The ion exchanger in the BRITA Maxtra filter cartridge reduces the carbonate hardness, and so prevents the limescale deposits.