Which oxide does not react with calcium oxide (CaO)?

• A. Na2O
• B. SO2
• C. SO3
• D. CO2

Answer: (A)

Calcium oxide is a strong basic oxide, so it readily reacts with acidic oxides to form salts such as carbonates and sulfates. When a basic oxide meets an acidic oxide, the oxide ion from the base combines with the acid-derived species to give compounds like CaCO3 from CO2, or CaSO3/CaSO4 from SO2 or SO3.

Sodium oxide is itself a basic oxide. Two basic oxides don’t neutralize each other in the same way, so there isn’t a straightforward driving force or product for a reaction between CaO and Na2O under typical conditions. That makes sodium oxide the oxide that does not react with calcium oxide. The other oxides shown—CO2, SO2, and SO3—are acidic or acidic anhydrides and will react with CaO to form calcium carbonate or calcium sulfite/sulfate.