Water hardness — temporary (carbonate) hardness removal: Which treatment can remove temporary hardness of water most directly under household or plant conditions?

Introduction / Context:
Hardness in water arises from multivalent cations, chiefly calcium and magnesium. Temporary (carbonate) hardness is associated with bicarbonates and can be removed by processes that convert soluble bicarbonates to insoluble precipitates, while permanent hardness (sulphates, chlorides) requires chemical softening or ion exchange.

Given Data / Assumptions:

• Temporary hardness due to Ca(HCO3)2 and Mg(HCO3)2.
• No specialized softening plant assumed.
• Objective: identify the simplest effective removal method.

Concept / Approach:
Boiling shifts carbonate equilibria: CO2 is stripped, raising pH locally and precipitating CaCO3; magnesium precipitates as Mg(OH)2 at elevated temperatures and higher pH. Thus, boiling directly removes temporary hardness. Alum addition targets turbidity (coagulation), not hardness. Plain filtration/sedimentation without chemical reaction does not remove dissolved bicarbonates. Lime softening is an industrial process, but the option “adding lime only for permanent hardness” is misleading—lime primarily addresses carbonate hardness; however, the clearest, universally correct household/plant-level answer here is boiling.

Step-by-Step Solution:

Verification / Alternative check:
Lime soda softening also removes carbonate hardness but requires dosing control and equipment; boiling is the classical, direct method recognized in fundamentals questions.

Why Other Options Are Wrong:
Alum targets colloids, not hardness; filtration/sedimentation do not remove dissolved ions; the statement “adding lime only for permanent hardness” is incorrect in scope.

Common Pitfalls:
Confusing temporary vs permanent hardness; assuming coagulants remove dissolved ions; overlooking post-boil sludge removal.

Final Answer:
Boiling (driving off CO2 and precipitating CaCO3/Mg(OH)2)