In some cases it is desirable to soften hard water. Most detergents contain ingredients that counteract the effects of hard water on the surfactants. For this reason, water softening is often unnecessary. Where softening is practised, it is often recommended to soften only the water sent to domestic hot water systems to prevent or delay inefficiencies and damage due to scale formation in water heaters. A common method for water softening involves the use of ion-exchange resins, which replace ions like Ca by twice the number of mono cations such as sodium or potassium ions.
Washing soda (sodium carbonate, NaCO) is easily obtained and has long been used as a water softener for domestic laundry, in conjunction with the usual soap or detergent.
Water that has been treated by a water softening may be termed softened water. In these cases, the water may also contain elevated levels of sodium or potassium and bicarbonate or chloride ions.