Explain the terms hard water and soft water. Write a note on the
(ii) Calgon method for the removal of hardness of water.

Explanation

Water which gives ready and permanent lather with soap is called soft water.
Water which do not give ready and permanent lather with soap is called hard water.
Hardness of water is due to the presence of soluble chloride, sulphate and bicarbonate compounds of magnisium and calcium such as `MgCl_(2), MgSO_(4), Mg(HCO_(3))_(2), CaCl_(2), CaSO_(4), Ca(HCO_3)_(2)`.
(2) Calgon method : Sodium hexametaphosphate is commercially called as calgon.
When calgon is added to hard water it reacts with calcium and magnesium ions forming complex anions.
`Na_(2)[Na_(4)(PO_(3))_(6)] + 2Mg^(2+) rarr Na_(2)[Mg_(2)(PO_3)_(6)] + 4Na^(+)`
`Na_(2)[Na_(4)(PO_(3))_(6)] + 2Ca^(2+) rarr Na_(2)[Ca_(2)(PO_3)_(6)] + 4Na^(+)`
Due to the formation of the complex the `Mg^(2+)` and `Ca^(2+)` become inactive and cannot react with soap. So the water can give good lather. This method is used only for laundry process.