Question

How can the permanent hardness of water be removed? Give proper chemical equation in support of your answer.

Answer 1

• Permanent hardness of water is the hardness that cannot be removed by boiling which is usually due to the presence of calcium sulphate and magnesium sulphate in water which does not precipitate out as temperature increases.
• There are certain chemical methods to remove the permanent hardness of water which are,
• Treating it with washing soda- Washing soda is added to the hard water it combines with chloride salts of calcium and magnesium present in the water, the reaction is $\underset{\mathrm{calcium}\mathrm{chloride}}{\underset{}{{\mathrm{CaCl}}_2}}+\underset{\mathrm{sodium}\mathrm{carbonate}}{\underset{}{{\mathrm{Na}}_2{\mathrm{CO}}_3}}\to \underset{\mathrm{calcium}\mathrm{carbonate}}{\underset{}{{\mathrm{CaCO}}_3}}+\underset{\mathrm{sodium}\mathrm{chloride}}{\underset{}{2\mathrm{NaCl}}}$
• And when reacts with MgCl₂ to form MgCO₃ and 2NaCl
• Now the precipitates of CaCO₃ are removed by filtration which frees from Ca²⁺. After these ions are removed the water becomes soft.
• Permanent hardness of water is also removed by Calgon's method in which sodium hexametaphosphate (Na₆P₆O₁₈) is added to the water, and each of its molecules ionizes to give two Na⁺ ions and one complex anion.
• Another method is the Ion exchange method is used to remove the hardness of the water in this method we add a permutit called zeolite( sodium aluminium silicate) or (AlNa₁₂SiO₅) to the water which is insoluble in water.
• When we add this to water the ion exchange process starts between it and the Ca²⁺ or Mg²⁺ ions. In this way, water becomes free from Ca²⁺ or Mg²⁺ ions and no longer remains hard.
• Synthetic resins method in which we use synthetic ion exchange resins which are insoluble in water, an exchange between RNa⁺ and Ca²⁺ or Mg²⁺ ion occurs. The process frees water from the ions.