Which reagent, when boiled with hard water, can remove both temporary (bicarbonate) and permanent (non-bicarbonate) hardness by precipitating calcium and magnesium salts?

Introduction / Context:
Hardness removal is a classic water-treatment topic. Temporary hardness arises from bicarbonates of calcium and magnesium, while permanent hardness is due to chlorides, sulfates, and nitrates of these ions. Different reagents target different fractions. Identifying a reagent that can address both types when heated clarifies laboratory and field dehardening practices.

Given Data / Assumptions:

• Objective: convert dissolved Ca2+/Mg2+ into insoluble precipitates.
• Boiling conditions or hot contact with the reagent are available.
• No ion exchange resins are used (chemical precipitation approach).

Concept / Approach:
Sodium carbonate (washing soda) supplies carbonate ions that precipitate calcium as CaCO3 and, after subsequent reactions on heating, can also help precipitate magnesium as MgCO3 and Mg(OH)2. In contrast, lime (calcium hydroxide) primarily removes temporary hardness by converting bicarbonates to carbonates; permanent hardness typically requires soda ash addition. Thus, boiling hard water with sodium carbonate can address both fractions more comprehensively among the listed choices.

Step-by-Step Solution:

Verification / Alternative check:
Lime–soda ash softening in municipal practice employs both Ca(OH)2 and Na2CO3; among the single reagents listed, sodium carbonate is the more generally effective precipitant for both hardness types under hot conditions.

Why Other Options Are Wrong:

• Calcium hydroxide: Best for temporary hardness; often needs soda ash for permanent hardness.
• Calcium oxide: Quicklime that forms Ca(OH)2; same limitation as lime alone.
• Calcium carbonate: Sparingly soluble; not an effective reagent for removal.

Common Pitfalls:
Expecting complete deionization; chemical softening reduces hardness but does not remove all dissolved solids like ion exchange or RO.

Final Answer:
sodium carbonate