Explain the terms hard water and soft water. Write a note on the
(i) Ion - exchange method.

Explanation

Hard water : Water does not give lather readily with soap is called hard water.
`to` Hard water contains hardness. This hardness is due to presence of Ca, Mg soluble salts.
`to` Presence of Ca, Mg - bicarbonates causes temporary hardness.
`to` Presence of Ca, Mg.- chlorides, sulphates causes permanent hardness.
Soft water : Water which give lather immediately with soap is called soft water.
(or) Water which is free from soluble salts of Ca, Mg is called soft water.
i) lon - Exchange method:
`to` This method is useful to remove the permanent hardness of water.
This method is also named as permutil (or) zeolite process.
`to` Permutit is the artificial zeolite. i.e. sodium aluminium orthosilicate. `(Na_(2)Al_(2)Si_(2)O_(8)xH_(2)O(or)NaAlSiO_(4))`
`to` Permutit is written in short form as Naz.
`to` When permutit is added to hard water, the following ion-exchange reactions takes place.
`2Naz_((s))+Ca_((aq))^(+2)toCaz_(2(s))+2Na_((aq))^(+)`
`2Naz_((s))+Mg_((aq))^(+2)toMgz_(2(s))+2Na_((aq))^(+)`
`toCaz_(2)andMgz_(2)` are called as exhausted permutit. These are regenerated to permutit by the treatment with brine solution.
`Caz_(2(s))+2Na_((aq))^(+)to2Naz_((s))+Ca_((aq))^(+2)`
ii) Calgon process :
`to` Calgon is, sodium hexametaphosphate. `[Na_(6)P_(6)O_(18)(or)(NaPO_(3))_(6)]`
`to` Calgon doesnot precipitate the Ca (or) Mg - salts but removes `Ca^(+2)andMg^(+2)` ions from water.
`to` The removal of `Ca^(+2)(or)Mg^(+2)` ions from water may takes place either by adsorption (or) by complex formation.
Reactions :