Which oxides are amphoteric?

• A. -Na2O, MgO
• B. CO2, SO2
• C. H2O
• D. Al2O3, ZnO

Answer: (D)

Amphoteric oxides can behave as acids in basic environments and as bases in acidic environments, dissolving or reacting to form species appropriate to either medium. Aluminum oxide and zinc oxide are classic examples because they react with acids to form salts and water, and in basic medium they dissolve to give hydroxo-complexes.

In acidic conditions, both oxides act like bases and are consumed to yield their respective metal ions in solution: Al2O3 reacts with strong acids to form aluminum salts, while ZnO reacts with acids to form zinc salts. In basic conditions, they show acidic character and dissolve by forming aluminates or zincates, such as [Al(OH)4]− and [Zn(OH)4]2−, respectively. This dual reactivity is what makes them amphoteric.

The other options don’t fit this dual behavior. Oxides that are primarily basic, like sodium oxide or magnesium oxide, react with acids but not with bases to form soluble hydroxo complexes under ordinary conditions. The listed acidic oxides (like CO2 and SO2) mainly react with water to form acids and with bases to give salts, but they don’t exhibit the same reversible amphoteric behavior as the metal oxides shown. Water itself isn’t classified as an amphoteric oxide in this context, even though it can act as an acid or base.