In water softening, what fraction of hardness does the sodium–zeolite (cation-exchange) process remove under normal operation?

Introduction / Context:
Hardness in water arises from divalent cations, primarily calcium and magnesium. Different softening processes target different hardness fractions. The sodium–zeolite process (ion exchange) is widely used in domestic and industrial softening systems; understanding its scope avoids misapplication and sets expectations for downstream scaling control.

Given Data / Assumptions:

• Zeolite or synthetic resin in sodium form exchanges Na+ for Ca2+ and Mg2+.
• Temporary hardness relates to bicarbonates; permanent hardness to chlorides/sulfates.
• Regeneration uses brine (NaCl) to restore Na+ form.

Concept / Approach:
Cation exchange is non-selective with respect to the associated anion. Whether calcium or magnesium is present as bicarbonate (temporary hardness) or as sulfate/chloride (permanent hardness), the resin removes the cation and replaces it with sodium. Therefore, both temporary and permanent hardness are reduced to very low levels until resin exhaustion.

Step-by-Step Solution:

Verification / Alternative check:
Softened water analyses show near-zero Ca2+/Mg2+ until breakthrough; alkalinity/anions pass unchanged, confirming cation-specific removal independent of hardness type.

Why Other Options Are Wrong:

• Only temporary or only permanent: Misstates the broad cation-exchange action.
• Dissolved gases: Not a hardness issue and not removed by zeolite softening.

Common Pitfalls:
Confusing TDS with hardness; ion exchange swaps cations but does not lower total dissolved solids significantly, as Na+ increases while Ca2+/Mg2+ decrease.

Final Answer:
both temporary and permanent hardness of water