This reference gives the test and the positive result for each common gas in SPM Chemistry, from the squeaky pop of hydrogen to limewater turning milky with carbon dioxide. Use it to identify a gas from what you observe.
Identifying a gas is a standard practical skill, and each common gas has a simple confirmatory test with a clear positive result. The exam expects you to know the test, carry it out correctly, and describe the result in the exact words the marking scheme wants. This reference lists the gas tests you meet in SPM Chemistry, arranged the way the DSKP presents them, so you can name a gas from the observation and write a full-mark answer.
Scope note
The table below holds the standard KSSM gas tests for Form 4 and Form 5. Learn the test and the positive result together, because a question usually describes one and asks for the other, or asks you to identify the gas from a described observation. Describe colour and change precisely, since the mark depends on the wording.
Gas tests and results
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen, H2 | Bring a lighted wooden splint to the mouth of the tube | A squeaky "pop" sound |
| Oxygen, O2 | Bring a glowing wooden splint to the gas | The glowing splint relights |
| Carbon dioxide, CO2 | Bubble the gas through limewater | Limewater turns milky (cloudy white) |
| Ammonia, NH3 | Hold moist red litmus paper in the gas | Red litmus turns blue; pungent smell |
| Chlorine, Cl2 | Hold moist blue litmus paper in the gas | Blue litmus turns red, then is bleached white |
| Sulfur dioxide, SO2 | Hold paper soaked in acidified potassium manganate(VII) | The purple solution is decolourised |
| Water vapour | Hold dry cobalt(II) chloride paper in the vapour | Blue cobalt chloride paper turns pink |
How to use the table
Work from the observation to the gas. A colourless gas that gives a squeaky pop with a lighted splint is hydrogen; one that relights a glowing splint is oxygen; one that turns limewater milky is carbon dioxide. A gas with a pungent smell that turns moist red litmus blue is ammonia, while a pale green gas that bleaches moist litmus is chlorine. Sulfur dioxide decolourises acidified potassium manganate(VII). For water, blue cobalt chloride paper turning pink, or white anhydrous copper(II) sulfate turning blue, confirms it. Always test a gas that is colourless first by smell and splint before using a reagent.
Worked example
A colourless gas is collected and turns limewater milky. Which gas is it, and what causes the change? The gas is carbon dioxide. Limewater is calcium hydroxide solution, and it turns milky because a white precipitate of insoluble calcium carbonate forms: Ca(OH)2(aq) + CO2(g) gives CaCO3(s) + H2O(l). If more carbon dioxide is bubbled through, the milkiness can clear as soluble calcium hydrogencarbonate forms, which is a useful extra detail.
Common mistakes to avoid
- Swapping the splint tests; a glowing splint relights in oxygen, a lighted splint pops with hydrogen.
- Writing that chlorine only turns litmus red; it turns it red then bleaches it white.
- Saying limewater turns “white” rather than “milky” or “cloudy”.
- Forgetting that ammonia is the only common gas here that turns red litmus blue.
How we help
Our online one-to-one teachers rehearse each test and its exact wording, so you can identify a gas and describe the result the way the marking scheme rewards. Lessons are in English, one to one, from RM50 per hour, with a paid one-hour trial. Because gas tests appear in both written and practical papers, a firm command of this table turns identification questions into quick, secure marks across Form 4 and Form 5.
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