Question

In the ion-exchange process for water treatment, hard water is softened by:

• A. Precipitation of salts
• B. Exchange of $Ca^{2+}$ and $Mg^{2+}$ ions with $Na^{+}$ or $H^{+}$ ions
• C. Removal of suspended particles
• D. Boiling the water

Hint:

Logic Tip: The name of the process gives it away! "Ion-exchange" implies that ions are being swapped. Since hardness is caused by calcium and magnesium, you must exchange those specific ions for others like sodium or hydrogen to fix the problem.

Answer 1

The Correct Option is B

Concept:
Ion exchange is a reversible chemical process where undesirable ions in a liquid are replaced by more desirable ions through a solid medium (resin). In water treatment, this is specifically used to remove the minerals that cause water to be "hard".

Step 1:
Hardness in natural water is primarily caused by the presence of dissolved calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) salts.

Step 2:
To soften the water, the concentration of these specific divalent cations ($Ca^{2+}$ and $Mg^{2+}$) must be effectively reduced or removed.

Step 3:
The hard water is passed through a bed containing ion-exchange resins that are pre-loaded with monovalent ions like sodium ($Na^{+}$) or hydrogen ($H^{+}$).

Step 4:
As the hard water flows through the resin, the resin releases its $Na^{+}$ or $H^{+}$ ions into the water and captures the $Ca^{2+}$ and $Mg^{2+}$ ions from the water in their place.

Step 5:
The resulting treated water is "soft" because it now contains sodium or hydrogen ions instead of the scale-forming calcium and magnesium ions.