Question

Zero hardness of water is achieved by

• A. Using lime soda process
• B. Excess lime treatment
• C. Ion exchange method
• D. Using excess alum dosage

Hint:

Remember:
• Lime-soda process $\rightarrow$ Soft water.
• Ion exchange process $\rightarrow$ Demineralized water with almost zero hardness.

Answer 1

The Correct Option is C

Concept: Hardness of water is mainly caused by the presence of calcium (\(\mathrm{Ca^{2+}}\)) and magnesium (\(\mathrm{Mg^{2+}}\)) ions. Different softening methods remove hardness to different extents.
• Lime-soda process removes only temporary and part of permanent hardness.
• Excess lime treatment removes temporary hardness.
• Ion exchange process removes both temporary and permanent hardness almost completely. Hence, zero hardness can be achieved only by the ion exchange process.

Step 1: Understand the ion exchange process. In the ion exchange method, \[ \mathrm{Ca^{2+}},\ \mathrm{Mg^{2+}} \] are replaced by \[ \mathrm{H^+}\ \text{or}\ \mathrm{Na^+} \] using suitable ion exchange resins. This removes practically all hardness-producing ions.

Step 2: Identify the correct method. Therefore, \[ \boxed{\text{Ion exchange method produces water with nearly zero hardness}.} \] Hence, the correct option is \[ \boxed{(C)\;\text{Ion exchange method}.} \]