AQA GCSE Chemistry
AQA GCSE Chemistry 4.8.2 Identification of Common Gases Exam Questions
Free AQA GCSE Chemistry exam questions for 4.8.2 Identification of Common Gases, with a mark scheme for every pack. Covers the laboratory tests and positive results for hydrogen, oxygen, carbon dioxide and chlorine.
4.8.2 Identification of Common Gases exam questions
2 question packs of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About AQA GCSE Chemistry 4.8.2 Identification of Common Gases
AQA GCSE Chemistry 4.8.2 is a short but high-value topic: the four standard tests for common gases. You need to know the test for hydrogen, oxygen, carbon dioxide and chlorine and exactly what a positive result looks like. These tests are assessed on Paper 2 as part of section 4.8 Chemical analysis, but they appear across the whole course, for example in electrolysis (4.4.3), reactions of acids (4.4.2) and carbonate tests (4.8.3). Examiners award marks only for precise wording, so learn each test as a matching pair of method and result.
What you need to know
- Hydrogen Hold a burning splint at the open end of a test tube of the gas. Hydrogen burns rapidly with a squeaky pop.
- Oxygen Insert a glowing splint into a test tube of the gas. The splint relights in oxygen.
- Carbon dioxide Shake or bubble the gas through limewater (an aqueous solution of calcium hydroxide). Carbon dioxide turns limewater milky (cloudy).
- Chlorine Hold damp litmus paper in the gas. Chlorine bleaches the litmus paper and it turns white (blue litmus may turn red first).
- Where tests are used Hydrogen from acids with metals or the cathode in electrolysis; oxygen from the anode or decomposing hydrogen peroxide; carbon dioxide from acids with carbonates; chlorine from electrolysis of chloride solutions.
Common mistakes in 4.8.2 Identification of Common Gases
- Using a glowing splint for hydrogen or a lit splint for oxygen. Hydrogen needs a lighted (burning) splint; oxygen needs a glowing splint.
- Using dry litmus paper for chlorine. The paper must be damp.
- Writing that limewater 'goes cloudy' without naming limewater, or saying the gas turns limewater 'clear'. Carbon dioxide turns limewater milky or cloudy.
- Saying hydrogen 'pops'. Examiners usually accept 'squeaky pop'; a vague 'it explodes' may not score.
- Writing 'litmus turns red' as the only result for chlorine. The key result is that it is bleached white.
Try three 4.8.2 Identification of Common Gases exam-style questions
1. Describe the test for oxygen and give the result. (2 marks)
Show the mark scheme answer
Insert a glowing splint into the gas; the splint relights.
2. A gas is produced when dilute hydrochloric acid is added to a solid. The gas turns limewater milky. Name the gas and suggest what type of compound the solid was. (2 marks)
Show the mark scheme answer
Carbon dioxide; a carbonate (metal carbonate).
3. Aqueous sodium chloride is electrolysed. Describe how you could identify the gas at each electrode. (4 marks)
Show the mark scheme answer
Positive electrode gas (chlorine): damp litmus paper; is bleached; negative electrode gas (hydrogen): lighted splint; burns with a squeaky pop.
4.8.2 Identification of Common Gases FAQs
What does AQA GCSE Chemistry 4.8.2 cover?
It covers the tests for hydrogen, oxygen, carbon dioxide and chlorine and the observations that show each gas is present.
Which paper is 4.8.2 on?
It is part of section 4.8 Chemical analysis, assessed on AQA GCSE Chemistry Paper 2, though gas tests can also be used in Paper 1 questions on electrolysis and acids.
How do I remember the gas tests?
Learn them as pairs: hydrogen, burning splint, squeaky pop; oxygen, glowing splint, relights; carbon dioxide, limewater, milky; chlorine, damp litmus, bleached.
