Chromatography

    OCR
    GCSE

    Chromatography is a physical separation technique relying on the differential distribution of components between a mobile phase and a stationary phase. Separation occurs because substances possess varying solubilities in the mobile phase and varying degrees of attraction to the stationary phase. This process facilitates the distinction between pure substances and mixtures, as well as the qualitative identification of components via Retention Factor (Rf) values. Mastery requires understanding the dynamic equilibrium established during separation and the precise experimental conditions necessary for valid results.

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

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating the stationary phase is the paper and the mobile phase is the solvent
    • Credit responses that explain separation occurs because substances have different solubilities in the mobile phase and different attractions to the stationary phase
    • Award 1 mark for calculating the Rf value using the formula: distance moved by substance ÷ distance moved by solvent front
    • Credit the explanation that a pure substance produces a single spot, whereas an impure substance produces multiple spots
    • Award 1 mark for stating the baseline must be drawn in pencil because ink would dissolve in the solvent and interfere with results

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly calculated the Rf value, but ensure you measure to the centre of the spot for precision"
    • "Good identification of the pure substance. To improve, explain *why* it is pure (single spot) using the correct terminology"
    • "You mentioned the spots move at different speeds — refine this by discussing 'solubility in the mobile phase' and 'attraction to the stationary phase'"
    • "Remember that the baseline must be pencil; explain the consequence of using ink (it dissolves/smears) to secure the explanation mark"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating the stationary phase is the paper and the mobile phase is the solvent
    • Credit responses that explain separation occurs because substances have different solubilities in the mobile phase and different attractions to the stationary phase
    • Award 1 mark for calculating the Rf value using the formula: distance moved by substance ÷ distance moved by solvent front
    • Credit the explanation that a pure substance produces a single spot, whereas an impure substance produces multiple spots
    • Award 1 mark for stating the baseline must be drawn in pencil because ink would dissolve in the solvent and interfere with results

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When calculating Rf values, always measure to the centre of the spot and ensure the final value is less than 1
    • 💡Use specific terminology: refer to the 'mobile phase' and 'stationary phase' rather than just 'water' and 'paper' to access higher marks
    • 💡In 'Suggest' questions regarding unknown substances, compare Rf values directly to known standards provided in the data; a match indicates identity

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Drawing the start line in ink, which dissolves in the solvent and ruins the chromatogram
    • Placing the solvent level above the start line, causing the spots to wash away into the solvent rather than travel up the paper
    • Calculating Rf values greater than 1 by incorrectly dividing the solvent distance by the spot distance
    • Describing separation based on 'speed' or 'density' rather than the correct principles of solubility and attraction to phases

    Key Terminology

    Essential terms to know

    Mobile phase and stationary phase dynamics
    Calculation and interpretation of Rf values
    Distinction between pure substances and mixtures
    Experimental setup and error analysis

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Explain
    Describe
    Suggest
    Interpret

    Practical Links

    Related required practicals

    • •{"code":"PAG C4","title":"Separation techniques","relevance":"Investigation of the composition of inks using paper chromatography"}

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