What gas is evolved when acids react with carbonates such as calcium carbonate?

What gas is evolved when acids react with carbonates such as calcium carbonate?

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

Answer: (B)

When an acid meets a carbonate, the carbonate ion reacts with the acid’s hydrogen ions to form carbon dioxide and water. The carbonate acts as a base and is protonated, so CO3^2− + 2H+ gives CO2 + H2O. The CO2 is a gas, so you see bubbles or effervescence as it escapes from the solution. This fits the common equation CaCO3 + 2HCl → CaCl2 + CO2 + H2O, showing the carbonate turning into carbon dioxide and water. Hydrogen gas would come from a reaction where a metal reacts with an acid, not from carbonates. Nitrogen and methane aren’t produced in this acid–carbonate reaction, so they aren’t involved here.

When an acid meets a carbonate, the carbonate ion reacts with the acid’s hydrogen ions to form carbon dioxide and water. The carbonate acts as a base and is protonated, so CO3^2− + 2H+ gives CO2 + H2O. The CO2 is a gas, so you see bubbles or effervescence as it escapes from the solution. This fits the common equation CaCO3 + 2HCl → CaCl2 + CO2 + H2O, showing the carbonate turning into carbon dioxide and water.

Hydrogen gas would come from a reaction where a metal reacts with an acid, not from carbonates. Nitrogen and methane aren’t produced in this acid–carbonate reaction, so they aren’t involved here.