Identification of Common GasesAQA GCSE Chemistry: Revision notes
Section 1
How do we identify hydrogen gas in the laboratory?
Hydrogen is identified using the burning splint test. When a lit splint is placed at the mouth of a test tube containing hydrogen gas, the hydrogen ignites with a characteristic squeaky pop sound. This distinctive noise occurs because hydrogen burns rapidly in the presence of oxygen, producing a small explosion.
- Collect hydrogen gas in a test tube (typically produced by reacting a metal with dilute acid)
- Light a wooden splint with a Bunsen burner
- Blow out the flame so the splint glows
- Place the glowing splint at the mouth of the test tube
- Listen for the squeaky pop sound
The squeaky pop is caused by the rapid combustion of hydrogen: 2H₂ + O₂ → 2H₂O.
Examiners want you to describe the sound the splint makes — 'squeaky pop' — not just say 'it burns'. Use these exact words in your answer for full marks.
Students often say 'light a splint' instead of 'use a glowing or burning splint'. The splint must already be alight when you test for hydrogen, otherwise you won't hear the pop.
Section 2
How do we identify oxygen gas in the laboratory?
Oxygen is identified using the glowing splint test. When a glowing (not burning) splint is placed into a test tube containing oxygen gas, the splint relights and burns more brightly. This occurs because oxygen supports combustion — it allows materials to burn more vigorously than they would in air.
- Collect oxygen gas in a test tube (typically produced by decomposing hydrogen peroxide with manganese dioxide catalyst)
- Light a wooden splint with a Bunsen burner
- Blow out the flame so only the tip glows (no flame)
- Insert the glowing splint into the test tube
- Observe the splint reigniting with a bright flame
Oxygen does not burn itself; instead, it allows other materials to burn more readily. This is why the splint reignites rather than producing a sound like hydrogen.
The key difference from the hydrogen test is that the splint must be glowing, not burning. If you use a lit splint, you'll get a false positive. Examiners specifically test whether you know this distinction.
Think of oxygen as a 'fuel accelerant' — like pouring petrol on a fire. The splint was already trying to burn (glowing), but oxygen makes it burn much more vigorously.
Section 3
How do we identify carbon dioxide gas using limewater?
Carbon dioxide is identified using limewater (a solution of calcium hydroxide). When carbon dioxide gas is bubbled through limewater, the solution turns milky or cloudy white. This colour change is caused by the formation of calcium carbonate, an insoluble white precipitate.
- Pour limewater into a test tube or beaker
- Bubble the gas through the solution using a delivery tube (or draw the gas through using a pipette)
- Observe the colour change from clear to milky white
- If excess carbon dioxide continues to be passed through, the white precipitate may dissolve again, forming soluble calcium hydrogen carbonate
The chemical equation for the initial reaction is: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
This is the most reliable test for carbon dioxide and is commonly used in the laboratory.
Always describe the change as 'turns milky' or 'becomes cloudy white' — these are the mark scheme terms. Say 'limewater' not 'lime solution' for precision.
If a student collects gas from reacting hydrochloric acid with calcium carbonate and tests it with limewater, the limewater turns milky white. This confirms the gas is carbon dioxide. If they continue bubbling for longer, the white precipitate may disappear because excess CO₂ dissolves the CaCO₃ — but this second change is not required for identification.
Section 4
How do we identify chlorine gas using damp litmus paper?
Chlorine is identified using damp litmus paper. When chlorine gas comes into contact with damp litmus paper, the paper bleaches and turns white (or loses its colour). This occurs because chlorine has strong bleaching properties — it chemically destroys the colour in the dye on the litmus paper.
- Prepare a piece of litmus paper and dampen it slightly with water
- Expose the damp litmus paper to chlorine gas (do not inhale; chlorine is toxic)
- Observe the colour fading or the paper turning white
- Red litmus paper will turn white; blue litmus paper will also turn white
Note: Chlorine dissolves slightly in water to form hypochlorous acid, which is responsible for the bleaching effect. The key observation is the loss of colour on the litmus paper.
Safety warning: Chlorine gas is toxic and corrosive. This test should only be performed in a fume hood or well-ventilated area under teacher supervision.
The litmus paper must be damp — dry litmus paper will not turn white in chlorine gas. Examiners always check whether you specify this detail. State clearly: 'damp litmus paper turns white' or 'bleaches'.
Students sometimes confuse this test with the behaviour of acidic gases (like CO₂ or SO₂) which turn litmus paper colours due to pH changes. Chlorine is different — it actually destroys the colour through bleaching, so the paper becomes colourless white, not a different colour.
Chlorine works like household bleach on a coloured fabric — it chemically destroys the dye molecules, making them colourless. The litmus paper is like the fabric; the colour disappears completely rather than changing to another colour.
Section 5
What is the difference between the four gas tests?
Each gas has a distinctive test and observation that allows positive identification. The four tests are fundamentally different in method and observation:
| Gas | Test Method | Observation | Key Feature |
|---|---|---|---|
| Hydrogen | Burning splint | Squeaky pop sound | Hydrogen ignites rapidly |
| Oxygen | Glowing splint | Splint relights | Oxygen supports combustion |
| Carbon dioxide | Bubble through limewater | Solution turns milky white | Forms insoluble precipitate |
| Chlorine | Damp litmus paper | Paper turns white/bleaches | Chlorine has bleaching power |
Each test relies on a different chemical property of the gas:
- Hydrogen is highly flammable
- Oxygen supports combustion
- Carbon dioxide forms a precipitate with limewater
- Chlorine has bleaching properties
When identifying an unknown gas, you should perform the appropriate test based on these properties. In an exam, you may be given a gas and asked to describe how to identify it.
In exam questions, be precise about the observation you expect. Don't just say 'it changes' — say exactly what you see (e.g. 'the splint relights', 'the solution turns milky white', 'the paper bleaches').
Must Know
- Hydrogen test: Use a burning splint at the mouth of the test tube; observe a squeaky pop sound as hydrogen ignites rapidly in air.
- Oxygen test: Use a glowing splint (blown out so no flame remains) and insert into the gas; observe the splint relights because oxygen supports combustion.
- Carbon dioxide test: Bubble the gas through limewater; observe the solution turns milky white due to formation of insoluble calcium carbonate precipitate.
- Chlorine test: Use damp litmus paper (the paper must be wet); observe the paper turns white or bleaches due to chlorine's bleaching properties.
- Each test is based on a unique chemical property of the gas, so there is no confusion between them.
- Always describe observations using exact mark scheme terminology: 'squeaky pop', 'relights', 'turns milky', 'bleaches/turns white'.
That's the notes covered.
Carry on to the next subtopic.