Softening chemistry: Permanent hardness (non-carbonate hardness) in water can be removed most directly by adding which chemical reagent during lime–soda softening or similar treatment?

Introduction:
Permanent hardness stems from calcium and magnesium salts of strong acids (chlorides, sulfates, nitrates) that do not decompose on boiling. Chemical softening targets these ions to form insoluble carbonates or hydroxides removable by clarification/filtration. Understanding reagent roles is essential in water-treatment design.

Given Data / Assumptions:

• Permanent hardness must be precipitated or exchanged.
• Chemicals should convert dissolved Ca2+/Mg2+ into insoluble forms.
• We focus on common reagents used in conventional softening.

Concept / Approach:
In lime–soda softening, lime (Ca(OH)2) removes temporary hardness by converting bicarbonates, while soda ash (Na2CO3) provides carbonate ions that precipitate calcium as CaCO3 from non-carbonate salts. Magnesium often requires additional lime to form Mg(OH)2. Thus, soda ash is the key chemical for tackling permanent hardness directly.

Step-by-Step Solution:
Identify the target: Ca2+/Mg2+ from chlorides/sulfates/nitrates.Select a reagent that supplies CO3^2− to precipitate CaCO3.Soda ash (Na2CO3) fulfills this role in softening processes.

Verification / Alternative check:
Standard water-treatment texts show calculation steps where soda ash dosage is based on non-carbonate hardness equivalents, confirming its specific role.

Why Other Options Are Wrong:

• Lime: Primarily addresses temporary hardness; also aids Mg removal but not the main reagent for Ca from permanent hardness.
• Potassium permanganate: Oxidant for iron/manganese; not a softener.
• Sodium bicarbonate: Adds alkalinity; does not precipitate hardness effectively.
• Alum: Coagulant; not a hardness-precipitating agent.

Common Pitfalls:
Confusing alkalinity adjustment (lime) with carbonate supply (soda ash); correct dosing requires hardness speciation.

Final Answer:
soda ash