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When a reaction bubbles, fizzes or pops, the gas given off is often the clue to what has happened. Being able to identify a gas from a simple test is a core practical skill, and it appears throughout C4 — confirming the hydrogen from a metal-and-acid reaction, the carbon dioxide from a carbonate, or the chlorine from electrolysis. There are four gas tests you must know precisely: the method you carry out and the result you look for. This lesson, part of Topic C4 of OCR Gateway Science A, sets out the four tests for hydrogen, oxygen, carbon dioxide and chlorine.
By the end of this lesson you should be able to describe the test and the positive result for each of the four gases, state how each gas can be produced, and identify an unknown gas from a described observation.
Each test has a distinctive positive result, summarised here and explained in detail below.
| Gas | Test (method) | Positive result |
|---|---|---|
| Hydrogen (H2) | Hold a lighted splint at the mouth of the tube | Squeaky pop |
| Oxygen (O2) | Insert a glowing splint into the tube | The splint relights |
| Carbon dioxide (CO2) | Bubble the gas through limewater | Limewater turns milky / cloudy |
| Chlorine (Cl2) | Hold damp blue litmus paper in the gas | Paper turns red then is bleached white |
Exam Tip: For full marks always give both parts — the test and the result. "Use a splint" is not enough; you must say lighted or glowing, and state the squeaky pop or relighting.
To test for hydrogen, hold a lighted (burning) splint at the mouth of the test tube. If the gas is hydrogen, it burns rapidly with a sharp "squeaky pop". The pop is the sound of the small amount of hydrogen burning explosively in air:
2H2+O2→2H2O
Hydrogen is produced whenever a metal above hydrogen reacts with a dilute acid (metal + acid → salt + hydrogen), for example Zn+H2SO4→ZnSO4+H2, and also at the cathode in many electrolysis experiments.
Exam Tip: The hydrogen test uses a lighted splint (it must already be burning). A common slip is to say a glowing splint — that is the oxygen test. Hydrogen = lighted splint, squeaky pop.
To test for oxygen, take a glowing splint (a splint that has been lit and then blown out so it is still glowing red but not flaming) and put it into the gas. If the gas is oxygen, the extra oxygen makes the splint relight (burst back into flame). Oxygen supports combustion, so it reignites the smouldering wood.
Oxygen is produced, for example, by the decomposition of hydrogen peroxide (often catalysed by manganese(IV) oxide), 2H2O2→2H2O+O2, and at the anode in the electrolysis of many aqueous solutions.
Exam Tip: Oxygen relights a glowing splint — it does not give a pop. Keep the pairings straight: lighted splint + pop = hydrogen; glowing splint + relights = oxygen.
To test for carbon dioxide, bubble the gas through limewater (a solution of calcium hydroxide). If the gas is carbon dioxide, the limewater turns milky (cloudy white). The cloudiness is a fine precipitate of insoluble calcium carbonate:
Ca(OH)2+CO2→CaCO3+H2O
Carbon dioxide is produced whenever a carbonate reacts with an acid (carbonate + acid → salt + water + carbon dioxide), for example CaCO3+2HCl→CaCl2+H2O+CO2, by thermal decomposition of carbonates, and in combustion and respiration.
Exam Tip: Limewater turns milky only with carbon dioxide, not with any gas. The positive result is "limewater turns milky / cloudy" — say milky, not just "changes".
To test for chlorine, hold a piece of damp blue litmus paper in the gas. If the gas is chlorine, the paper first turns red (chlorine forms an acidic solution) and is then quickly bleached white. The decisive observation is the bleaching — chlorine destroys the dye, leaving the paper white.
Chlorine is produced at the anode when a chloride solution (such as brine) is electrolysed, and when concentrated hydrochloric acid is oxidised. Chlorine is a toxic, choking gas, so the test is done with only a small amount and, in industry, the gas is handled in a fume cupboard — its bleaching action on the damp litmus is the very property exploited in household bleach and in sterilising drinking water.
Exam Tip: The key word for chlorine is bleaches: damp litmus turns red then white. If you only say "turns red", you have described an acid, not specifically chlorine — the bleaching to white is what identifies it.
In the chlorine test the litmus paper must be damp, not dry. Chlorine itself does not change the colour of dry paper; it has to dissolve in the moisture on the paper to form an acidic, bleaching solution before it can act. A common exam slip is to forget the word "damp" — a dry paper would show no change even with chlorine present. The same principle applies to other indicator-paper tests: the water lets the gas dissolve and react.
It is worth remembering why each positive result happens, because "explain" questions ask for the reason, not just the observation. The squeaky pop is the sound of hydrogen burning rapidly (almost explosively) in the air at the mouth of the tube. Oxygen relights a splint because it actively supports combustion — more oxygen means a hotter, more vigorous burn, enough to reignite smouldering wood. Limewater turns milky because dissolved carbon dioxide reacts with the calcium hydroxide to make a fine suspension of insoluble calcium carbonate, the same white solid as chalk. And chlorine bleaches because it chemically destroys the coloured dye in the litmus.
Exam Tip: Pair each result with its reason: hydrogen burns (pop); oxygen supports combustion (relights); carbon dioxide forms insoluble calcium carbonate (milky); chlorine destroys the dye (bleaches). The reason is what turns a "describe" answer into an "explain" answer.
It helps to know a typical laboratory source for each gas, because exam questions often describe the reaction that produces the gas and then ask you to identify it.
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