Which of the following acids would readily react with CaCO3 to liberate CO2?

Analyze the question: We need the acid that reacts with calcium carbonate (CaCO3) to release carbon dioxide readily.

Concept: Carbonates react with acids to form a salt, water and CO2:

CaCO3 + 2HX → CaX2 + H2O + CO2

The key problem with CaCO3 is that the salt formed must be soluble. If the salt is insoluble, it coats the surface of the carbonate and stops the reaction, so the reaction is not "ready" or sustained.

Checking each option:

• A. CH3COOH (ethanoic acid): CaCO3 + 2CH3COOH → (CH3COO)2Ca + H2O + CO2. It does react, because calcium ethanoate is soluble. But it is a weak acid, so the reaction is slower and less vigorous.
• B. H2SO4: CaCO3 + H2SO4 → CaSO4 + H2O + CO2. CaSO4 is only sparingly soluble and forms a coating on the marble or limestone, so the reaction quickly slows and stops.
• C. H2SO3 (sulphurous acid): It is a weak, unstable acid. It is hardly stronger than carbonic acid, and CaSO3 is insoluble, so it is not a good choice.
• D. HNO3 (nitric acid): CaCO3 + 2HNO3 → Ca(NO3)2 + H2O + CO2. It is a strong acid, and calcium nitrate is very soluble, so no coating forms. The reaction goes on vigorously and readily.

It is a strong acid that forms a soluble calcium salt, so CO2 is liberated readily and continuously.

Tip: For "readily reacts with a carbonate", think of two things: a strong acid and a soluble salt product. All nitrates are soluble, which makes HNO3 (and HCl) reliable choices. Be careful with H2SO4 and CaCO3, as the insoluble CaSO4 layer stops the reaction.