Question

Permanent hardness cannot be removed by

• A. Using washing soda
• B. Calgon’s method
• C. Clark’s method
• D. Ion exchange method

Answer 1

The Correct Option is C

Permanent hardness in water is caused by the presence of dissolved salts like calcium sulfate (CaSO₄) and magnesium sulfate (MgSO₄). This type of hardness cannot be removed by simple precipitation methods. Let's review the methods:

• Washing soda (Na₂CO₃): This is used to remove temporary hardness by precipitating calcium and magnesium ions as carbonates. However, it is not effective for permanent hardness, which requires more complex methods.
• Calgon's method: Calgon, or sodium hexametaphosphate, is used to remove both temporary and permanent hardness by forming soluble complexes with calcium and magnesium ions.
• Clark's method: Clark's method is used to remove only temporary hardness by adding lime (Ca(OH)₂) to precipitate calcium carbonate (CaCO₃). This method is not effective for permanent hardness, as it does not deal with the soluble sulfates (CaSO₄, MgSO₄) causing permanent hardness.
• Ion exchange method: This method involves exchanging calcium and magnesium ions with sodium or hydrogen ions, effectively removing both temporary and permanent hardness.

The correct answer is (C) : Clark’s method.