2020 O Level Chem P1 Q3 — Qualitative Analysis / Tests for Ions and Gases Solution

Answer: B (3 items). Work out the standard gas test for each gas and count the distinct items needed. Ammonia turns damp red litmus paper blue. Chlorine bleaches damp litmus paper, so damp blue litmus paper is used to show bleaching cleanly. Hydrogen gives a squeaky pop with a lighted wooden splint. Oxygen relights a glowing wooden splint.

So the items required are: damp red litmus paper, damp blue litmus paper, and a wooden splint. The same splint can be lit for hydrogen or made to glow for oxygen, so only one splint is counted. That gives a minimum of three items.

Two items would not be enough to distinguish all four gases unambiguously, and four or five items are more than necessary. Hence the minimum number of reagents needed is 3.