Which gas is evolved when a carbonate reacts with an acid in the insoluble solid reaction?

• A. Carbon dioxide
• B. Oxygen
• C. Hydrogen
• D. Nitrogen

Answer: (A)

When a carbonate reacts with an acid, it releases carbon dioxide gas. The solid carbonate plus the acid produces a salt and water, and the carbonate breaks down to form CO2, which bubbles out as gas.

For example, a solid calcium carbonate reacting with acid like hydrochloric acid gives calcium chloride, water, and carbon dioxide: CaCO3 + 2HCl → CaCl2 + CO2 + H2O. The CO2 is the gas you observe as effervescence.

That gas is carbon dioxide, not oxygen, hydrogen, or nitrogen, because the reaction pathway specifically converts the carbonate (CO3^2−) into CO2 when the acid supplies H+. A quick test is to bubble the gas through limewater; CO2 turns limewater cloudy.