Softening Permanent Hardness — Which chemical treatment can soften permanent hard water by sequestration/complexation?

Introduction:
Permanent hardness stems from non-carbonate salts (for example, CaSO4, MgCl2) that do not decompose on boiling. Such waters can be softened by precipitation (lime–soda), ion exchange, or sequestration using polyphosphates. This question focuses on a classical sequestration method used historically in domestic and small-scale applications.

Given Data / Assumptions:

• Target is calcium and magnesium ions contributing to permanent hardness.
• We are considering chemical addition routes rather than thermal softening.
• Answer choices include common salts used in water conditioning.

Concept / Approach:

Sodium hexametaphosphate (often marketed as “Calgon”) complexes Ca2+ and Mg2+, converting them into soluble, non-precipitating forms that do not form scale or soap scum. This is sequestration, not removal; the hardness ions remain in solution but are “tied up.” For true removal, precipitation or ion exchange would be required.

Step-by-Step Solution:

Verification / Alternative check:

Detergent formulations historically included polyphosphates to control hardness. Modern environmental constraints have reduced their use due to eutrophication concerns, but the chemistry remains valid.

Why Other Options Are Wrong:

A: Sodium silicate may aid corrosion control but is not a standard sequestering softener. B: Bicarbonate alters alkalinity but does not neutralize permanent hardness. D: Sodium phosphate variants can precipitate under some conditions but are not the classic sequestrant. E: Ferric chloride is a coagulant, not a softener.

Common Pitfalls:

Confusing “softening” by removal with “conditioning” by sequestration; Calgon softening is conditioning that prevents scale formation without reducing total dissolved hardness.

Final Answer:

Sodium hexametaphosphate (Calgon)