Water chemistry – temporary hardness removal: Which chemical addition can remove temporary hardness (bicarbonates of Ca^2+ and Mg^2+) from water by precipitation?

Introduction / Context:
Hardness in water arises from multivalent cations, primarily calcium and magnesium. Temporary hardness is associated with bicarbonates and can be removed by boiling or by chemical softening. This question focuses on the classic lime softening reaction used in water treatment.

Given Data / Assumptions:

• Temporary hardness species: Ca(HCO3)2 and Mg(HCO3)2.
• Goal: convert dissolved species into insoluble precipitates that can be settled/filtered.
• Ambient conditions typical of conventional water treatment basins.

Concept / Approach:
Adding calcium hydroxide [Ca(OH)2] raises pH and converts bicarbonates to carbonate, precipitating hardness as CaCO3 and Mg(OH)2. The net effect is removal of temporary hardness by forming low-solubility solids. Other salts listed either do not remove hardness or can increase it.

Step-by-Step Solution:
For Ca(HCO3)2: Ca(OH)2 + Ca(HCO3)2 → 2 CaCO3(s) + 2 H2O.For Mg(HCO3)2: Ca(OH)2 + Mg(HCO3)2 → MgCO3 + CaCO3 + 2 H2O; further: MgCO3 + Ca(OH)2 → Mg(OH)2(s) + CaCO3(s).Precipitates (CaCO3, Mg(OH)2) settle, reducing temporary hardness.Thus, calcium hydroxide is the appropriate reagent.

Verification / Alternative check:
Jar tests in water plants demonstrate turbidity rise from precipitates and reduced alkalinity post-lime dosing, consistent with hardness removal by precipitation.

Why Other Options Are Wrong:
Calcium carbonate: sparingly soluble; does not drive precipitation when added directly.

Calcium chloride: adds Ca^2+, increasing hardness.

Sodium bicarbonate: adds bicarbonate alkalinity, potentially increasing temporary hardness.

Common Pitfalls:

• Confusing temporary vs permanent hardness; soda ash (Na2CO3) targets permanent hardness.
• Overdosing lime leading to excessive pH and post-stabilization needs.

Final Answer:
Calcium hydroxide (lime)