How to test for common gases in the lab

Identifying a gas is one of the most testable practical skills in SPM Chemistry. It appears directly in Paper 3 and inside qualitative analysis, where the gas released tells you which ion is present. The marks are won not by naming the gas but by describing the test and the exact observation precisely. This guide gives you both for every common SPM gas, plus the tricky pairs that catch students out.

How to phrase a gas test for marks

Every gas-test answer has two parts: what you do (the test) and what you see or hear (the observation). “It turns limewater milky” is only half an answer; the full version is “bubble the gas through limewater, the limewater turns milky (cloudy white).” Two habits keep you accurate:

• Moisten the litmus or test paper before use. The gas has to dissolve in the film of water to act on the paper, so a dry paper may show nothing.
• Describe the change, not just the end state. “Bleached” and “turned red then colourless” earn more than “went white.”

The common gases at a glance

Gas	Test	Observation
Hydrogen, H₂	Bring a lighted wooden splint to the mouth of the tube	Burns with a squeaky “pop” sound
Oxygen, O₂	Insert a glowing wooden splint	The splint relights (rekindles)
Carbon dioxide, CO₂	Bubble the gas through limewater	Limewater turns milky (chalky white)
Ammonia, NH₃	Hold moist red litmus paper in the gas	Red litmus turns blue
Chlorine, Cl₂	Hold moist blue litmus paper in the gas	Blue litmus turns red, then is bleached white
Sulfur dioxide, SO₂	Hold acidified potassium dichromate(VI) paper in the gas	Orange paper turns green

Notes on each test

Hydrogen is the only common gas that gives the “pop.” Use a lighted splint, not a glowing one, hydrogen needs a flame to ignite. The pop is the sound of a small amount of hydrogen burning quickly in air.

Oxygen supports combustion, so a splint that is only glowing bursts back into flame. The key word is “relights”; a lighted splint simply continuing to burn is not proof.

Carbon dioxide turns limewater, a solution of calcium hydroxide, milky because insoluble calcium carbonate forms. This test links straight to reactions such as the thermal decomposition of carbonates and metal carbonate plus acid.

Ammonia is the only common gas that is alkaline, so it is the one gas that turns moist red litmus blue. It also has a sharp, choking smell and forms dense white fumes when a rod dipped in concentrated hydrochloric acid is held near it.

The tricky pairs

Two comparisons trip students up, so learn them deliberately.

Chlorine versus sulfur dioxide. Both are acidic and both can decolourise litmus, so you must be precise. Chlorine is a greenish-yellow gas that turns moist blue litmus red and then bleaches it white. Sulfur dioxide is colourless and is best confirmed by turning acidified potassium dichromate(VI) from orange to green, because it is a reducing agent. If you only say “bleaches litmus,” you have not separated the two.

Ammonia versus everything else. Because ammonia is the sole alkaline gas here, “turns moist red litmus blue” is a complete identification on its own. No other common gas does this, so it is a gift when it appears.

Common mistakes to avoid

• Using a dry test paper, always write “moist” red or blue litmus.
• Confusing the splint tests, lighted splint for hydrogen (pop), glowing splint for oxygen (relights).
• Stopping at “bleaches” for chlorine without the “turns red first,” or forgetting the dichromate test for sulfur dioxide.
• Saying limewater “goes white” instead of the expected “milky” or “cloudy.”

Gas tests reward clean, memorised phrasing, so drill them as flashcards until the observation comes out word-perfect. You can revise the full set on the gas tests reference, and see where they fit into salt analysis in the qualitative analysis of anions experiment. If practical observations are where you lose marks, often just imprecise wording, that is quick to fix one-to-one. Our online lessons run in English from RM50 an hour, with a paid one-hour trial, rehearsing exam-ready observations against real Paper 3 tasks.