Temporary Hardness on Boiling — When temporarily hard water (bicarbonate hardness) is boiled, which of the following species is formed/released as part of the decomposition?

Introduction:
Temporary hardness arises from bicarbonates of calcium and magnesium. Heating or boiling decomposes bicarbonates, removing hardness by precipitating carbonates and releasing carbon dioxide gas. Recognizing the reaction products is important for distinguishing temporary from permanent hardness behaviors.

Given Data / Assumptions:

• Feed water contains Ca(HCO3)2 and/or Mg(HCO3)2.
• Boiling drives off CO2 and shifts equilibria.
• No chemical additives are considered (thermal softening only).

Concept / Approach:

On boiling, bicarbonate decomposes: Ca(HCO3)2 → CaCO3(s) + CO2(g) + H2O. Thus, the process both forms a solid precipitate (calcium carbonate) and releases carbon dioxide gas. The key identifiable gaseous product is CO2, which can be observed as effervescence during boiling.

Step-by-Step Solution:

Verification / Alternative check:

Kettle scaling (CaCO3) and bubbling CO2 are common household evidence of this reaction. Analytical tests confirm lowered alkalinity after boiling.

Why Other Options Are Wrong:

A: Calcium bicarbonate is the reactant, not the product. B: Calcium sulfate relates to permanent hardness; boiling doesn’t remove it. C: Hydrogen chloride is not involved. E: Sodium nitrate is unrelated.

Common Pitfalls:

Assuming boiling removes all hardness; permanent hardness (sulfates/chlorides) remains and needs chemical treatment or ion exchange.

Final Answer:

Carbon dioxide (gas released)