A student is trying to identify the gases produced during an experiment. Describe the tests a student would carry out to identify carbon dioxide gas and oxygen gas, including the expected results for each test.

OCR GCSE Chemistry A: Gateway Science (J248) — C4.2 Identifying the products of chemical reactions · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

During a chemistry experiment, two gases are collected separately in test tubes. The student needs to confirm which gas is carbon dioxide and which is oxygen.

Model answer (4 marks)

1. Test for CO₂: bubble the gas through limewater (Ca(OH)₂ solution). 2. Result: the limewater turns cloudy or milky, indicating CaCO₃ precipitation.
3. Test for O₂: place a glowing splint into the test tube containing the gas.
4. Result: the splint relights, showing the presence of oxygen.

Examiner tips

  • Use the exact wording ‘bubble the gas through limewater’ and ‘glowing splint’ to match the mark scheme.
  • Show the expected change (cloudy/milky and relight) for each test.
  • Keep the answer concise – 4 points, one sentence per test and result.

Common mistakes

  • Mixing up the tests (e.g., using limewater for oxygen).
  • Forgetting to mention the change in limewater (cloudy).
  • Using a non‑glowing splint or not describing the relight.

Mark scheme (4 marks)

  1. Test for carbon dioxide: bubble the gas through limewater (calcium hydroxide solution)
  2. Result for carbon dioxide: limewater turns cloudy / milky
  3. Test for oxygen: place a glowing splint into the test tube of gas
  4. Result for oxygen: the glowing splint relights

Key terms in this question

carbon dioxide · oxygen

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