Zeolite (Ion-Exchange) Softening — The process removes both temporary and permanent hardness. In this method, the calcium and magnesium in water are effectively removed as:

Introduction:
Water hardness arises from dissolved calcium and magnesium salts. The zeolite (also called sodium-zeolite) softening process removes hardness via ion exchange, not by bulk precipitation. Recognizing the correct removal mechanism helps distinguish it from lime–soda softening (a precipitation process).

Given Data / Assumptions:

• Sodium-form zeolite (Na-Z) exchanges Na+ for Ca2+/Mg2+ in hard water.
• Process targets both temporary (bicarbonate) and permanent (sulfate/chloride) hardness.
• Regeneration is later accomplished with brine (NaCl) to restore Na-Z.

Concept / Approach:

In ion exchange, calcium and magnesium ions are captured on the solid zeolite matrix as Ca-Z and Mg-Z while sodium ions are released to the water. No insoluble carbonate, sulfate, or chloride precipitates in the bulk solution during this step; instead, the ions are immobilized on the resin/zeolite. Hence, they are “removed as insoluble zeolite forms.”

Step-by-Step Solution:

Verification / Alternative check:

Operationally, the bed exhausts as exchange sites become Ca/Mg-loaded; regeneration with NaCl brine reverses the reaction, restoring Na-Z and releasing Ca/Mg in the brine waste.

Why Other Options Are Wrong:

A, C, D: These describe precipitation mechanisms typical of lime–soda treatment, not zeolite exchange. E: No gaseous product removes hardness in this process.

Common Pitfalls:

Assuming all softening involves precipitation. Ion exchange is distinct and often used as a polishing step after clarification/filtration.

Final Answer:

Insoluble zeolites (Ca/Mg form) retained on the ion-exchange matrix