What is observed when carbonate ions are treated with dilute acid?

• A. Effervescence and carbon dioxide produced.
• B. No reaction.
• C. Oxygen evolution.
• D. Hydrogen gas.

Answer: (A)

When carbonate ions meet acid, they react to form carbon dioxide and water. The carbonate ion accepts hydrogen ions, turning into CO2 and H2O: CO3^2- + 2H+ → CO2 + H2O. Since CO2 is a gas, bubbles form and rise from the solution—this effervescence is the observable signal of the reaction. Oxygen would require a different process, and hydrogen gas would come from other reactions, so those gases aren’t produced here. If the solution shows bubbling, that confirms CO2 is being released.