Temporary hardness removal by boiling: hardness will be removed by simple boiling if it is primarily due to which dissolved salts of calcium?

Introduction / Context:
Hardness in water is commonly classified as temporary (carbonate) and permanent (non-carbonate). Understanding which salts cause which type of hardness dictates the appropriate treatment method at household or plant scale.

Given Data / Assumptions:

• Boiling precipitates bicarbonates as carbonate scale.
• Permanent hardness salts remain dissolved after boiling.

Concept / Approach:
Temporary hardness arises from bicarbonates of calcium and magnesium. On boiling, bicarbonate ions decompose, releasing carbon dioxide and precipitating carbonates, thus reducing hardness. Sulphates, chlorides, and nitrates of calcium or magnesium constitute permanent hardness and are not removed by boiling.

Step-by-Step Solution:

Verification / Alternative check:
Household experience: kettles develop carbonate scale when boiling hard water, indicating precipitation of carbonates from bicarbonates.

Why Other Options Are Wrong:

• Sulphates, chlorides, nitrates of calcium (b–d) persist on boiling; they require softening (lime-soda, ion exchange).
• (e) is incorrect because (a) is valid.

Common Pitfalls:
Confusing carbonate hardness (temporary) with non-carbonate hardness (permanent). Always identify the anion to choose the treatment.

Final Answer:
Calcium bicarbonates