Testing for Gases

    OCR
    GCSE

    Qualitative analysis requires precise experimental procedures to identify unknown gases based on unique chemical properties. Candidates must distinguish between tests for hydrogen, oxygen, carbon dioxide, and chlorine using specific reagents and observation techniques. The chemistry underpinning these tests includes rapid combustion, oxidation, and precipitation reactions, specifically the formation of insoluble calcium carbonate in the presence of carbon dioxide. Accurate reporting of positive results, such as the bleaching of damp litmus or the relighting of a glowing splint, is essential for valid identification.

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    Objectives
    3
    Exam Tips
    4
    Pitfalls
    5
    Key Terms
    6
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for identifying the test for hydrogen as applying a **lit splint**
    • Award 1 mark for stating the positive result for hydrogen is a **squeaky pop**
    • Award 1 mark for describing the test for carbon dioxide as **bubbling** gas through **limewater**
    • Award 1 mark for stating limewater turns **cloudy** or **milky** (formation of white precipitate)
    • Award 1 mark for identifying the test for chlorine using **damp blue litmus paper** which is **bleached** (turns white)
    • Award 1 mark for identifying the test for ammonia using **damp red litmus paper** which turns **blue**

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the gas, but you didn't specify that the litmus paper must be **damp**"
    • "Make sure to distinguish between the 'test' (glowing splint) and the 'result' (relights) — don't mix them up"
    • "Good recall of the limewater test. Can you explain *why* it turns cloudy (formation of calcium carbonate)?"
    • "You confused the hydrogen and oxygen tests — remember 'Pop' goes with the 'Lit' splint"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for identifying the test for hydrogen as applying a **lit splint**
    • Award 1 mark for stating the positive result for hydrogen is a **squeaky pop**
    • Award 1 mark for describing the test for carbon dioxide as **bubbling** gas through **limewater**
    • Award 1 mark for stating limewater turns **cloudy** or **milky** (formation of white precipitate)
    • Award 1 mark for identifying the test for chlorine using **damp blue litmus paper** which is **bleached** (turns white)
    • Award 1 mark for identifying the test for ammonia using **damp red litmus paper** which turns **blue**

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When asked to describe a test, provide both the **action** (what you do) and the **observation** (what you see) to ensure full marks
    • 💡For carbon dioxide, use the phrase 'bubble through' or 'pass through' limewater; simply adding it is insufficient for a gas test
    • 💡Remember that ammonia is the only alkaline gas required by the specification; link this property to the damp red litmus turning blue

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing the splint type: stating 'glowing splint' for hydrogen or 'lit splint' for oxygen
    • Omitting the state of the litmus paper for chlorine or ammonia tests; it must be described as **damp** for the gas to dissolve and react
    • Describing the result for chlorine merely as 'turns red' (acidic property) rather than 'bleaches white' (the specific identifying property)
    • Stating 'add limewater' without specifying the method of delivery (bubbling/passing gas through)

    Key Terminology

    Essential terms to know

    Identification of Hydrogen (Squeaky Pop test)
    Identification of Oxygen (Relighting glowing splint)
    Identification of Carbon Dioxide (Limewater precipitation)
    Identification of Chlorine (Bleaching damp litmus)
    Chemical equations for positive test results

    Likely Command Words

    How questions on this topic are typically asked

    Describe
    State
    Identify
    Explain
    Suggest

    Practical Links

    Related required practicals

    • •{"code":"PAG C8","title":"Analysis and identification of ions and gases","relevance":"Direct assessment of gas testing techniques and observations"}

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