Gas tests: identifying six gases in IGCSE Chemistry
Match ammonia, carbon dioxide, chlorine, hydrogen, oxygen and sulfur dioxide to the required test without confusing splints, litmus or decolourisation.
How do you identify a gas in IGCSE Chemistry?
Match a gas's observed reaction to the correct test. In your answer, name both the test material and the exact change you would observe. Here are the six gas tests for Cambridge IGCSE Chemistry 0620.
| Gas | Test material | Result |
|---|---|---|
| Ammonia (NH₃) | Moist red litmus paper | The paper becomes blue. |
| Carbon dioxide (CO₂) | Limewater | A milky appearance develops. |
| Chlorine (Cl₂) | Moist litmus paper | The paper loses its colour (bleaching). |
| Hydrogen (H₂) | A lit splint | A popping sound is heard. |
| Oxygen (O₂) | A glowing splint | The splint starts burning again. |
| Sulfur dioxide (SO₂) | Acidified aqueous potassium manganate(VII) | The solution's purple colour disappears. |
The splints are the easy mix-up: hydrogen pops with a lighted splint; oxygen relights a glowing one. These are revision observations for supervised laboratory tests. Use the given candidate gases; one result does not identify every possible mixture.
Syllabus reference: Cambridge Chemistry 0620 syllabus, 2026–2028 · gas tests, page 48
Keep going: explanation and practice ↓Core and Extended · Chemistry 0620, 2026–2028. Interpret the supplied laboratory observations on paper; actual chemical tests require an appropriate supervised laboratory method.
Before you begin
- Recognise NH₃, CO₂, Cl₂, H₂, O₂ and SO₂ formulas.
- Distinguish a lighted splint from a glowing splint and damp litmus from dry paper.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which test matches carbon dioxide here?
What you will learn
- Describe the six required gas tests with their positive observations.
- Choose the right test within a supplied candidate set and distinguish observation from inference.
Six required gas tests
Try recalling the result, then open this comparison. Read the test and observation together. Actual testing belongs to an appropriate supervised laboratory method.
Open the comparison reference
Ammonia · NH₃
- Test
- Damp red litmus paper.
- Observation
- Turns blue.
Carbon dioxide · CO₂
- Test
- Limewater.
- Observation
- Turns milky.
Chlorine · Cl₂
- Test
- Damp litmus paper.
- Observation
- Is bleached.
Hydrogen · H₂
- Test
- Lighted splint in the specified supervised method.
- Observation
- A pop.
Oxygen · O₂
- Test
- Glowing splint in the specified supervised method.
- Observation
- The glowing splint relights.
Sulfur dioxide · SO₂
- Test
- Acidified aqueous potassium manganate(VII).
- Observation
- Purple reagent becomes colourless.
Use the supplied candidate set and conditions. One observation does not establish every ion in a mixture or certify purity.
Pair the test material with the actual response
Ammonia NH₃ turns damp red litmus paper blue. Carbon dioxide CO₂ turns limewater milky. Chlorine Cl₂ bleaches damp litmus paper. The damp condition and the observed change matter: 'litmus changes colour' alone is incomplete and does not distinguish ammonia's blue result from chlorine's bleaching.
Hydrogen H₂ gives a pop with a lighted splint in the specified supervised gas-test method. Oxygen O₂ relights a glowing splint. A glowing splint is different from an already burning lighted splint; keep the test condition and response together. These observations are interpreted here on paper, with appropriate gas handling and controlled ignition tests belonging to supervised laboratory work.
Use the correct purple reagent and limit the inference
Sulfur dioxide SO₂ turns acidified aqueous potassium manganate(VII), KMnO₄, from purple to colourless in the required test. The reagent's identity, aqueous/acidified conditions and change are needed, rather than merely saying a coloured liquid faded. Gas SO₂ and the sulfite ion SO₃²⁻ are different species even though their specified tests can both involve this reagent.
Use the supplied candidate set and controlled observations. One reaction should not be called a unique identification against every gas or every possible mixture. Do not deliberately smell an unknown gas, and do not generate or test hazardous gases outside an appropriate supervised procedure. A report should state the observed litmus, limewater, splint or reagent response first, then the supported candidate.
Pause and explain
Which pair correctly distinguishes the two splint tests?
Worked example
Three supplied gases give: damp red litmus turns blue; a glowing splint relights; and acidified aqueous KMnO₄ becomes colourless. Assign candidates from the six required gases.
Show the worked solution
- Damp red litmus turning blue matches ammonia NH₃.
- Relighting a glowing splint matches oxygen O₂.
- Purple-to-colourless acidified KMnO₄ matches sulfur dioxide SO₂ in the supplied candidate model.
Answer NH₃; O₂; SO₂ respectively, based on the stated tests and candidate set.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Give the test and observation for NH₃, CO₂ and Cl₂.
Give me a hint
State exactly what happens rather than only naming the paper or liquid.
Compare my reasoning
- NH₃ turns damp red litmus blue.
- CO₂ turns limewater milky.
- Cl₂ bleaches damp litmus paper.
NH₃: damp red litmus blue; CO₂: limewater milky; Cl₂: damp litmus bleached.
Look for these in your work
- All three tests include the response.
- Damp litmus and ammonia/chlorine differences are retained.
Give the test and observation for H₂, O₂ and SO₂.
Give me a hint
Keep lighted/glowing and the specified acidified reagent separate.
Compare my reasoning
- H₂ gives a pop with a lighted splint in the supervised test.
- O₂ relights a glowing splint.
- SO₂ changes acidified aqueous KMnO₄ from purple to colourless.
H₂: lighted splint/pop; O₂: glowing splint/relights; SO₂: acidified aqueous KMnO₄ purple to colourless.
Look for these in your work
- The two splint states are correct.
- The sulfur-dioxide reagent and colour change are specific.
A report says only 'gas X changed litmus' and 'gas Y reacted with a splint'. Explain what must be recorded before choosing among the six required gases.
Give me a hint
Incomplete observations can fit different tests.
Compare my reasoning
- For X, state whether the paper was damp/red and whether it turned blue or was bleached.
- For Y, state lighted or glowing and whether the response was a pop or relighting.
- Use those complete observations and the supplied candidate set; the vague report alone is insufficient.
Record litmus conditions and exact colour response, plus splint state and pop/relighting response; the vague descriptions do not identify X or Y.
Look for these in your work
- Missing test conditions are named.
- No unsupported identity is invented from incomplete evidence.
Common mistakes
- Swapping hydrogen's lighted splint with oxygen's glowing splint.
- Writing only 'litmus changes' without dampness and the resulting colour.
- Confusing CO₂ with CO or SO₂ with SO₃²⁻.
- Using deliberate smelling as the unknown-gas identification method.
What can you explain now?
Which gas relights a glowing splint in the required candidate set?
Compare with the explanation
Oxygen, O₂.
Hydrogen instead gives the specified pop with a lighted splint.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Recall all six gas-test pairs, then correct an incomplete report that omits dampness, colour change or the splint's starting state.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.