Permanent hardness of water can be removed by?

Analysis of the Question

The question asks for a method to remove permanent hardness from water. It is important to distinguish between temporary hardness (which can be removed by boiling) and permanent hardness (which requires chemical treatment).

Explanation of the Concept

• Temporary Hardness: Caused by the presence of calcium hydrogen trioxocarbonate(IV) or magnesium hydrogen trioxocarbonate(IV). This is easily removed by boiling, which precipitates the minerals as carbonates.
• Permanent Hardness: Caused by the presence of soluble calcium or magnesium chlorides or tetraoxosulphate(VI) salts. Because these salts do not decompose upon boiling, we must use chemical methods to precipitate the calcium or magnesium ions out of the solution.
• Methods for removing permanent hardness: • Adding washing soda (sodium trioxocarbonate(IV)). • Using ion-exchange resins (Permutit process). • Adding caustic soda (sodium hydroxide).

Step-by-Step Solution

1. Evaluate Option A (Filtration): Filtration only removes suspended solid particles; it cannot remove dissolved mineral salts that cause hardness.
2. Evaluate Option B (Adding slaked lime): Slaked lime ($Ca(OH)_2$) is used in the Clark’s process to remove temporary hardness, not permanent hardness.
3. Evaluate Option C (Adding caustic soda): Caustic soda ($NaOH$) reacts with the calcium or magnesium ions in the hard water to form insoluble precipitates (like $Ca(OH)_2$ or $Mg(OH)_2$), effectively removing the hardness. This is a standard chemical method for treating permanent hardness.
4. Evaluate Option D (Boiling): As established, boiling only removes temporary hardness. It has no effect on the chlorides or tetraoxosulphate(VI) salts responsible for permanent hardness.

Conclusion

Adding caustic soda is an effective chemical method for removing permanent hardness by precipitating the ions responsible for the hardness.

Exam Tip: Always remember the "Boiling Rule": If the question asks about temporary hardness, think "Boiling." If it asks about permanent hardness, think "Chemicals" (like washing soda or caustic soda) or "Ion Exchange."