2022 O Level Chem P1 Q28 — Reactivity Series / Tests for Gases Solution

Answer: D. Two facts to determine:

(i) Which metal is more reactive? A more reactive metal forms a more stable carbonate, so its carbonate decomposes more slowly. \(\text{ZCO}_3\) takes longer (266 s) than \(\text{XCO}_3\) (92 s) → \(\text{ZCO}_3\) is more stable → Z is more reactive than X. ✓

(ii) What test confirms the gas? The gas released by carbonate decomposition is \(\text{CO}_2\): \(\text{XCO}_3 \rightarrow \text{XO} + \text{CO}_2\). The standard test for \(\text{CO}_2\) is its reaction with limewater (\(\text{Ca(OH)}_2\)) to give a white precipitate of \(\text{CaCO}_3\). A glowing splint relighting tests for \(\text{O}_2\), not \(\text{CO}_2\).

Both "Z more reactive than X" and "white precipitate with limewater" match row D.