Topic 3 of 3
Classify an oxide by its reactions
Metal/non-metal character suggests a pattern, but exceptions matter.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
Classify an oxide by its reactions
Metal/non-metal character suggests a pattern, but exceptions matter.
| Class | Behaviour | Examples |
|---|---|---|
| Basic | Reacts with acids to form salt and water | CuO, MgO: generally metal oxides |
| Acidic | Reacts with bases to form salt and water | CO2, SO2: generally non-metal oxides |
| Amphoteric | Reacts with both acids and strong alkalis | Al2O3, ZnO |
| Neutral | Does not show typical acidic or basic oxide reactions | CO, NO; being a non-metal oxide does not guarantee acidity |
For example, CO2(g) + 2NaOH(aq) -> Na2CO3(aq) + H2O(l) with excess alkali shows acidic-oxide behaviour. CuO reacting with an acid shows basic behaviour. An insoluble oxide can still be acidic or basic: classification is not determined by whether it dissolves in water.
Check your understandingAn oxide reacts with hydrochloric acid and also with aqueous sodium hydroxide. How is it classified?Think it through, then reveal the answer
Amphoteric. Both reactions are needed to distinguish it from an oxide that is only basic or only acidic.