Saved Bookmarks
| 1. |
What different methods are used for softening the hard water ? Explain the principle used in each method ? (Write any two) |
|
Answer» Solution :Hard water can be softened by the following methods depending upon the nature of hardness. (a) Temporary hardness : (i) By boiling. It can be removed by merely boiling the water. Boiling decomposes the bicarbonates to give carbon DIOXIDE and insoluble carbonates, which can be removed by filtratoin. `Ca(HCO_(3))_(3)overset("heat")tounderset("white ppt.")(CaCO_(3))+H_(2)O+CO_(2)` `Mg(HCO_(3))_(2)overset("heat")tounderset("white ppt.")(MgCO_(3))+H_(2)O+CO_(2)` II. Clark's process : Temporary hardness can be removed by the addition of a calculated amount of lime, where magnesium carbonate or calcium carbonate is precipated. `Ca(HCO_(3))_(2)+Ca(OH)_(2)to2CaCO_(3)+2H_(2)O` `Mg(HCO_(3))_(2)+Ca(OH)_(2)toCaCO_(3)+MgCO_(3)+2H_(2)O` b. Permanent hardness : With sodium carbonate. On treatment with washing soda, `Ca^(2+)andMg^(2+)` in hard water are precipitated. The precipitate of the insoluble carbonates thus formed is removed by filtration. `Ca^(2+)+CO_(3)^(2-)tounderset("white ppt")(CaCO_(3)),Mg^(2+)+CO_(3)^(2-)toMgCO_(3)` The anion may remain in solution but in this form it is not dangerous. ii. Ion-exchange methode. The common SUBSTANCES used for this process is zeolite which is hydrated sodium aluminium silicate, `NaA1(SiO_(2))_(2).` The exchange occurs when on PASSING over the zeolite bed, sodium ions from zeolite are replaced by calcium and magnesium ions. Thus `2NaZe+Ca^(2+)to(Ze)_(2)+2Na^(+),2NaZe+Mg^(2+)to(Ze)_(2)Mg+2Na^(+)` When all the sodium ions of zeolite has replace, the zeolite is said to be EXHAUSTED. It can be regenerated by treatment with strong solution of sodium chloride. `2Na+(Ze)_(2)Cato2ZeNa+Ca^(2+)` |
|