Acid-base reaction producing acidic gas

Steps:

• Solution X reacts with a solid to evolve gas Y, indicating a decomposition or displacement reaction.
• Gas Y dissolves in water to form a solution with pH <7, so Y must be an acidic gas like CO2 or SO2.
• Common lab reaction: acid (X) added to carbonate solid evolves CO2 (Y).
• CO2 + H2O → H2CO3 (weak acid), confirming pH <7.

Why D is correct:

• Dilute HCl (X) + carbonate solid → CO2 (Y) + H2O + salt; CO2 forms carbonic acid in water (pH ≈5.6).

Why the others are wrong:

• A: NaOH (base) evolves NH3 (basic gas, pH >7 in water).
• B: Na2O (basic) evolves NH3 (basic gas, pH >7).
• C: HCl evolves NH3? No solid produces NH3 with acid; NH3 is basic (pH >7).

Final answer: D