Water which produce lather with soap is called soft water and which does not produce lather with soap solution easily is called hard water. Temporary hardness is due to presence of soluble bicarbonates of calcium and
magnesium and permanent hardness is due to the presence of chlorides and sulphate of calcium and magnesium. Temporary hardness can be removed by boiling or by addition of \(\mathrm{Ca}(\mathrm{OH})_{2}\). Permanent hardness can
be removed by the addition of washing soda ( \(\mathrm{Na}_{2} \mathrm{CO}_{3}\). \(10 \mathrm{H}_{2} \mathrm{O}\) ), adddition of sodium polymetaphosphate [calgon ( \(\mathrm{NaPO}_{3}\) ) \(_{n}\) ] or by ion exchange method).
