A soft water lathers easily with normal soap, but it is much more difficult to obtain a lather with hard water. Hardness is caused in two different ways. Permanent hardness is due to the presence of sulphates, chlorides and nitrates, usually as salts of calcium and magnesium, although iron and sodium may also be present. This type of hardness is described as ‘permanent’, as it cannot be removed by boiling the water. Temporary hardness can be removed by boiling and is due to the presence of bicarbonates, but loss of carbon dioxide results in conversion to carbonate and the familiar scale of hard water areas through formation of calcium carbonate. Some water supplies are treated with lime which reduces particularly temporary hardness by reducing the calcium and magnesium bicarbonate contents. Domestic water softening usually involves ion exchange resins which remove troublesome calcium and magnesium ions from water by absorption; the resins eventually become saturated and need to be reactivated, usually by passing common salt solution through the resin, releasing the absorbed ions and allowing them to be discharged to waste. Dishwashers often incorporate water softeners of this type with automatic reactification; it is important to maintain the level of softening salt, as well as the main washing detergent and the drying aid. Some water softening systems, particularly those used in laboratories to prepare deionised water, use cartridges which are exchanged for reactification.