Question

Permanent hardness of water can be removed by-

• A. Boiling
• B. Gel filtration
• C. Use of slaked lime
• D. Ion exchange process

Hint:

Industrial boilers and food processing equipment require fully softened (demineralized) water.
Using hard water causes the precipitation of calcium carbonate scale (scaling) on heat transfer surfaces, drastically reducing thermal efficiency and clogging pipes.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Water hardness is caused by dissolved divalent cations, primarily calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)).
- Temporary hardness: Caused by dissolved bicarbonates of calcium and magnesium, which can be easily precipitated and removed by simple boiling or adding slaked lime.
- Permanent hardness: Caused by dissolved sulfates and chlorides of calcium and magnesium, which cannot be removed by boiling.

Step 2: Detailed Explanation:
To remove permanent hardness, the calcium and magnesium ions must be physically removed or exchanged.
The ion exchange process utilizing synthetic resin columns is the most effective and widely used commercial method:
- Cation exchange resins replace the \(\text{Ca}^{2+}\) and \(\text{Mg}^{2+}\) ions with monovalent sodium (\(\text{Na}^+\)) or hydrogen (\(\text{H}^+\)) ions, completely softening the water.

Step 3: Final Answer:
Permanent hardness is removed by the Ion exchange process, which corresponds to option (D).