Carbonate ions treated with dilute acid produce which observation?

• A. Effervescence and carbon dioxide produced
• B. Color change to red
• C. Precipitate forms
• D. No reaction

Answer: (A)

Carbonate ions react with acids to release carbon dioxide gas. When the acid donates H+ to carbonate (CO3^2−), it forms carbonic acid, which quickly decomposes to CO2 and water. This gas evolution appears as effervescence or bubbling, which is the observed sign in dilute acid with carbonate ions. For example, a carbonate like CaCO3 reacts with dilute acid to give Ca2+ ions, CO2 gas, and H2O.

The other possibilities don’t fit this reaction. There isn’t a color change to red in this process, and no precipitate forms—the products are dissolved ions and carbon dioxide gas. And since gas bubbles appear, there is a reaction, so “no reaction” isn’t correct.