Identifying gases
Key idea: A gas test needs a stated method and a positive observation before naming the gas. Burning and glowing splints test different gases; litmus must be damp for ammonia and chlorine.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Read the test and the result for each of the six gases.
- Choose the litmus paper, both litmus results and the sulfur dioxide reagent.
- Read how to test a gas safely.
- Read what a negative result does and does not tell you.
- Choose the splint and the result for hydrogen and for oxygen.
- Read how to record what changed, and what the change shows.
- Name the gas that bleaches litmus and the gas that turns it blue.
- Decide what the pop shows, and what fizzing alone shows.
- Choose what the failed splint test shows, and why it may have failed.
- Rewrite the student's conclusion so it matches the result.
- Decide what the same colour change shows for a gas and for a solution.
- Read the common mistakes before the topic check.
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ContinueLearn the standard test for each of the six gases, then interpret the result using the possible gases and the test conditions. Record the test, the observation and then the inference. Fizzing only shows that a gas has formed; it does not identify it.
The six gas tests
| Gas | Test | Positive result |
|---|---|---|
| ammonia, NH₃ | hold damp red litmus paper in the gas | turns blue |
| carbon dioxide, CO₂ | bubble the gas through limewater | limewater turns milky |
| chlorine, Cl₂ | hold damp litmus paper in the gas | the paper is bleached |
| hydrogen, H₂ | hold a burning splint at the mouth of the tube | squeaky pop |
| oxygen, O₂ | insert a glowing splint | the splint relights |
| sulfur dioxide, SO₂ | bubble the gas through acidified aqueous potassium manganate(VII) | purple to colourless |
Swipe or scroll sideways to inspect the complete overview.
Swipe or scroll sideways to inspect the complete overview.
Ammonia makes damp red litmus blue, while chlorine bleaches damp litmus. These are different observations. Chlorine may briefly turn blue litmus red before bleaching it; the lasting positive result is the loss of colour.
The litmus tests need damp paper: the gas must dissolve in water on the paper before it can change the colour. Hydrogen needs a burning splint; oxygen needs a glowing one.
Check your understanding 1 Litmus and the sulfur dioxide test
Which litmus paper do you use, what do ammonia and chlorine each do to it, and which reagent does sulfur dioxide turn from purple to colourless?
Show answer
Use damp litmus paper. Ammonia turns damp red litmus blue; chlorine bleaches damp litmus. Sulfur dioxide turns acidified aqueous potassium manganate(VII) from purple to colourless.
Practical task 1 Test the gas, not the mixtureOptional
Hold damp litmus paper or a splint at the mouth of the test tube, not in the liquid. For carbon dioxide, lead the gas through a delivery tube into limewater. Test only a small sample, with your teacher, and keep flames away from any larger supply of hydrogen. Never smell a gas: ammonia, chlorine and sulfur dioxide are harmful.
A result that is negative does not identify another gas
A glowing splint that does not relight tells you the test did not show oxygen. It does not tell you the gas is hydrogen, or anything else. Air contains oxygen but does not relight a glowing splint, so a little oxygen mixed with air can fail the test too, and so can a splint that has stopped glowing.
So when a test gives no result, write “the test did not show oxygen”, not “there is no oxygen”, and check that the test was done properly.
Check your understanding 2 Two splint tests
A colourless gas has no effect on limewater or damp red litmus. It could be hydrogen or oxygen. Which splint test, and which result, shows each gas?
Show answer
A glowing splint that relights shows oxygen; a burning splint that gives a squeaky pop shows hydrogen. If neither test works, you have not identified the gas: a failed splint test is not evidence for any other gas.
Separate a gas result from a solution result
Of the six gases, only sulfur dioxide turns acidified potassium manganate(VII) from purple to colourless. But an unknown solution that gives the same colour change need not contain sulfur dioxide: any reducing agent can decolourise the reagent.
For example, when testing a gas from an acid–carbonate reaction, record that limewater became milky, then identify carbon dioxide. The gas itself has not turned white. If a gas turns damp red litmus blue, record the change in the paper, then identify ammonia.
Check your understanding 3 Blue or bleached?
Which gas bleaches damp litmus, and which turns damp red litmus blue?
Show answer
Chlorine bleaches the paper: it removes the colour. Ammonia turns damp red litmus blue. So “turns red litmus blue” is the ammonia result, never the chlorine one, even if chlorine turns the paper red for a moment first.
Check your understanding 4 A pop, or just fizzing?
A small sample of gas gives a squeaky pop with a burning splint. Which gas is it? What would fizzing alone have told you?
Show answer
Hydrogen: a squeaky pop with a burning splint is the hydrogen test. Fizzing only shows that a gas is forming; many reactions give off a gas, so fizzing cannot name it.
Check your understanding 5 A splint that does not relight
A little oxygen mixes with the air in a tube, and a glowing splint does not relight. What does the result show, and why might the test have failed?
Show answer
The test did not show oxygen. The oxygen may have been too dilute, or the splint may have stopped glowing, so the result does not mean there was no oxygen.
Check your understanding 6 Rewrite the conclusion
A reaction gives off a little oxygen, which mixes with the air already in the collecting tube. A glowing splint at the mouth of the tube does not relight, and the student writes: “No oxygen was produced.” Rewrite the conclusion so that it matches the evidence, and give one reason the test could have failed.
Show answer
“The glowing splint did not relight, so the test did not show oxygen.” The oxygen may have been too dilute, mixed with air, or the splint may have stopped glowing, so the result does not show that no oxygen was produced.
Check your understanding 7 The same colour change twice
Gas A, one of the six gases, turns acidified aqueous potassium manganate(VII) from purple to colourless. A few drops of an unknown solution B do the same to a fresh portion. What does each result show?
Show answer
Gas A is sulfur dioxide, because it is the only one of the six gases that decolourises the reagent. Solution B contains a reducing agent, but the colour change does not tell you which one.
Common mistakes
- Naming a gas from fizzing alone.
- Mixing up the splints: a lighted splint for hydrogen, a glowing splint for oxygen.
- Using dry litmus paper. The paper must be damp for ammonia and chlorine.
- Treating a negative result for one gas as proof that a particular other gas is present.
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Syllabus and review details
- SEC G3 Combined Science Chemistry component 2027 · 2027
Chemistry content structure and subject content, PDF pages 26–36
Last reviewed: