Water softening agents – Calgon: Calgon (sodium hexametaphosphate or related polyphosphate) is primarily used for the removal/control of which type of water hardness?

Introduction / Context:
Calgon is a trade name historically associated with sodium hexametaphosphate and related polyphosphates used as water conditioners. They act primarily by sequestering calcium and magnesium ions, preventing scale and 'permanent' hardness effects.

Given Data / Assumptions:

• Hardness ions: Ca^2+, Mg^2+.
• Permanent hardness: typically sulfates/chlorides that do not decompose upon boiling.
• Application: domestic and industrial water conditioning.

Concept / Approach:
Polyphosphates complex Ca^2+/Mg^2+ to form soluble complexes, reducing precipitation of carbonates/sulfates and mitigating hardness effects without necessarily removing ions by precipitation. This is particularly effective for permanent hardness species.

Step-by-Step Solution:
Recognize Calgon as a sequestrant (complexing agent) for Ca^2+, Mg^2+.Identify target: permanent hardness (sulfates/chlorides) not removed by boiling.Therefore, Calgon is used against permanent hardness, not to remove Na2CO3 or K2CO3.Select option describing permanent hardness control.

Verification / Alternative check:
Product literature and water-conditioning texts confirm polyphosphates inhibit scale by binding hard ions, supporting heat-exchanger protection and detergent performance.

Why Other Options Are Wrong:
Removing sodium/potassium carbonate is not Calgon’s role; these salts are already soluble and do not create hardness.

'None of these' is incorrect because combating permanent hardness is the conventional use.

Common Pitfalls:

• Confusing sequestration (keeping ions soluble) with true softening by precipitation/exchange.
• Ignoring polyphosphate reversion at high temperature/pH, which can reduce efficacy.

Final Answer:
Permanent hardness of water (sequestering Ca^2+ and Mg^2+)