The document discusses hard water and soft water. Hard water contains high levels of calcium and magnesium ions which can cause scaling. There are two types of hardness: permanent hardness caused by calcium/magnesium sulfates/chlorides and temporary hardness caused by calcium and bicarbonate ions. Temporary hardness can be removed by boiling. Softening hard water involves precipitating calcium ions using lime-soda or ion exchange processes. Soft water is better for soap lather but risks adding more sodium and dissolving metals from pipes.