Hydrogen burns with a pop at a lighted splint; oxygen relights a glowing splint; carbon dioxide turns limewater milky, CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l). Each gas has one confirmatory observation.
Identifying a gas by a simple confirmatory test is a skill examined across Form 4 and used in almost every practical. Three of the most common gases are hydrogen, oxygen and carbon dioxide, and each has a single, reliable test. The exam rewards the exact observation, the state of the splint, the precise colour change, so it is worth learning the wording carefully.
Test for hydrogen
Collect the gas and bring a lighted (burning) wooden splint to the mouth of the tube. Hydrogen burns explosively in air with a characteristic squeaky “pop” sound as it combines with oxygen to form water:
2H2(g) + O2(g) → 2H2O(l)
The “pop” is the small, quick combustion of the hydrogen. Note that a lighted splint is used, not a glowing one, this is what distinguishes the hydrogen test from the oxygen test.
Test for oxygen
Bring a glowing (nearly extinguished) wooden splint to the gas. Oxygen supports combustion strongly, so it relights the glowing splint, which bursts back into flame. No new compound needs to be written; the point is that oxygen rekindles a splint that has almost gone out. Using a glowing splint rather than a lighted one is essential, because a lighted splint would simply keep burning and prove nothing.
Test for carbon dioxide
Bubble the gas through limewater (calcium hydroxide solution). Carbon dioxide turns the limewater milky (cloudy white) because an insoluble white precipitate of calcium carbonate forms:
CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l)
If carbon dioxide continues to be bubbled through in excess, the milkiness disappears as the insoluble carbonate reacts further to form soluble calcium hydrogencarbonate:
CaCO3(s) + CO2(g) + H2O(l) → Ca(HCO3)2(aq)
For the standard test the answer is simply that carbon dioxide turns limewater milky; the disappearance in excess is a useful extra detail.
Putting the three together
These three tests often appear side by side, because a reaction may give off one of them and you must identify which. A metal with acid gives hydrogen (pop); the decomposition of hydrogen peroxide or heating some compounds gives oxygen (relights splint); a carbonate with acid, or combustion, gives carbon dioxide (limewater milky). Matching the gas produced to the correct test, and quoting the exact observation, is what earns the mark.
Common mistakes to avoid
State the state of the splint: lighted for hydrogen, glowing for oxygen, swapping them is the single most common error. For carbon dioxide, say limewater turns milky, not “white solid” alone, and give the equation if asked. Do not confuse the hydrogen “pop” with the oxygen “relight”. Include state symbols in the limewater equation, with the precipitate as CaCO3(s).
How it appears in the SPM exam
In Paper 1 (4541/1), objective items ask you to match a gas to its test or its observation. In Paper 2 (4541/2) you may name the gas released in a reaction and describe the test that confirms it, or write the limewater equation. In Paper 3 (4541/3), carrying out a gas test and recording the observation is a routine practical step. Learn the three tests as a set, pop, relight, milky, and pair each with its precise wording, and gas identification becomes quick and secure.
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