Gas tests: observations and inference
Key idea: Identify a gas from a specified test and its positive observation: hydrogen pops with a lighted splint, oxygen relights a glowing one, and carbon dioxide makes limewater milky.
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Guided lesson
Guided steps
- Choose the splint for the oxygen test.
- Match each test to its positive observation.
- Compare the starting splints and their positive changes.
- Write both gas tests and both positive observations.
- Follow the reagent changes, including excess carbon dioxide.
- Explain the milky-to-clear limewater sequence.
- Name the gas and explain its possible ion sources.
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ContinueA gas may be invisible even when its effect on a test reagent is clear. Identify it from the test and its positive observation, within the gases allowed by the question. A colourless gas, a few bubbles or a flame going out is not enough on its own.
Check your understanding 1 Choose the starting splint
You want to test whether a supplied gas is oxygen. Which splint should enter the gas?
Show answer
Use a glowing splint: the positive result is that it relights. A flame going out is not a positive oxygen or hydrogen identification.
Gas tests
| Gas | Test | Positive observation in the supplied notes |
|---|---|---|
| Ammonia, NH₃ | Damp red litmus paper | Paper turns blue |
| Carbon dioxide, CO₂ | Limewater | A white precipitate makes it milky; the precipitate dissolves with excess carbon dioxide |
| Chlorine, Cl₂ | Damp litmus paper | Paper is bleached |
| Hydrogen, H₂ | Lighted splint | A pop sound |
| Oxygen, O₂ | Glowing splint | Splint relights |
| Sulfur dioxide, SO₂ | Aqueous acidified potassium manganate(VII) | Purple solution becomes colourless |
The colours above belong to the paper or solution, not necessarily to the gas.
Record the observed change before naming the gas. For example, “damp red litmus turns blue” is an observation; “ammonia is present” is an inference.Some gases share test results, so use all the information given. Check the question’s possible gases and any stated unwanted reactions before claiming one identity.
Use gas tests only as specified in supervised laboratory work. Never identify a gas by smelling it. Chlorine and sulfur dioxide are hazardous to inhale; use supplied observations rather than improvising their production. In 9476, sulfur-dioxide tests are not required as a carry-out skill, although given results may need interpretation.
Lighted and glowing mean different starting states
A lighted splint has a flame. Hydrogen gives a pop when a small sample is tested by the specified method. A glowing splint has an ember but no flame. Oxygen supports combustion, so that glowing splint relights.
Do not substitute “the flame goes out” for either positive test. Many gases can extinguish a flame. The hydrogen pop and the oxygen relighting observation carry the discriminating evidence.
Exam-style question 1
Tell hydrogen and oxygen apart
Examination question
Two colourless gases are hydrogen and oxygen. State a test and positive observation for each gas. [4 marks]
Try this before viewing the solution
Show solution step by step
Test hydrogen
2 marksMethod
Apply a lighted splint to a small sample of gas.
Reason
Hydrogen combusts rapidly in air.Working
Answer
Positive observation: a pop sound.Test oxygen
2 marksMethod
Insert a glowing splint into a sample of gas.Reason
Oxygen supports combustion.Working
Answer
Positive observation: the glowing splint relights.
Self-mark with the mark scheme
Tick each mark point that your answer includes.
Self-mark each test and its matching positive observation separately.
Why damp litmus and limewater work
Ammonia dissolves in the moisture on damp litmus and makes it alkaline, turning red litmus blue. “Ammonia turns red” confuses the gas with the paper. Chlorine bleaches damp litmus; loss of the paper’s colour is different from an ordinary red-to-blue acid–base change.
Carbon dioxide reacts with limewater to precipitate calcium carbonate. With excess carbon dioxide, the precipitate can dissolve as soluble hydrogencarbonate is formed:
A final clear solution after prolonged passage of carbon dioxide does not mean that the initial positive milky observation was absent. Record the sequence, rather than only the final appearance.
Check your understanding 2 Interpret a sequence, not just its last state
A supplied unknown gas makes limewater turn milky. Continued passage makes the mixture clear again. The only possible gases are carbon dioxide and oxygen. Identify the gas and explain why the final clear appearance does not support “no reaction”.
Show answer
The gas is carbon dioxide. The first observation is formation of a white calcium carbonate precipitate. Excess carbon dioxide then dissolves it by forming soluble hydrogencarbonate. Oxygen does not explain that sequence in the supplied test. The final appearance alone loses the evidence from the first stage.
Check your understanding 3 Separate gas identity from the original ion
The possible gases are ammonia, carbon dioxide and chlorine. A supplied result says that warming an unknown solution with alkali and aluminium produces a gas that turns damp red litmus blue. Which gas fits this result? Can this observation alone identify the original ion?
Show answer
The damp-litmus result supports ammonia within the supplied gas set. It does not by itself identify the original ion: ammonium can release ammonia with alkali, while nitrate and nitrite can form ammonia during reduction. The test conditions, possible ions and any comparison results are needed for the ion inference. Anion tests develop that distinction.
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Syllabus and review details
- GCE A-Level H2 Chemistry 2027 · 9476-2027
9476 (2027), complete syllabus
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