Water softener. Domestic ion-exchange water softener in a kitchen cupboard. Water softeners are used in hard water areas. Hard water contains magnesium and calcium ions, with bicarbonate. These minerals cause the production of limescale when water is heated, and soap scum when detergents are used. The softener contains two tanks. One tank contains millions of tiny resin beads, which attract the magnesium and calcium ions from tap water and exchange them with sodium ions, thus softening the water. A second tank contains brine, which is used to wash the beads, removing the magnesium and calcium ions and replenishing the sodium ions.