Elements, compounds and mixtures

    OCR
    GCSE

    Matter is classified as elements, compounds, or mixtures based on atomic composition and the presence or absence of chemical bonds. Elements consist of a single type of atom, whereas compounds contain elements chemically bonded in fixed proportions; mixtures retain the individual properties of their constituent substances without chemical bonding. Mastery of this topic requires the application of physical separation techniques—filtration, crystallisation, simple and fractional distillation, and chromatography—and the analysis of melting and boiling point data to distinguish pure substances from impure mixtures.

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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 defining a pure substance as consisting of only one type of element or compound
    • Credit responses that identify a pure substance by a sharp, specific melting point, contrasting with the range observed in mixtures
    • Award 1 mark for calculating the Rf value as distance moved by spot divided by distance moved by solvent front
    • Candidates must explain that simple distillation separates a solvent from a solution because the solvent has a much lower boiling point
    • Award 1 mark for stating that the baseline in chromatography must be drawn in pencil to prevent it dissolving in the mobile phase

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the separation method, but you must explain *why* it works based on physical properties (e.g., boiling point)"
    • "Your Rf calculation is correct. To improve, explain what a higher Rf value implies about the substance's solubility in the mobile phase"
    • "Avoid using general terms like 'heater'; specify 'Bunsen burner' or 'electric heater' and explain safety considerations for flammable liquids"
    • "You stated the substance is pure. Now provide the evidence from the melting point graph to support this conclusion"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for defining a pure substance as consisting of only one type of element or compound
    • Credit responses that identify a pure substance by a sharp, specific melting point, contrasting with the range observed in mixtures
    • Award 1 mark for calculating the Rf value as distance moved by spot divided by distance moved by solvent front
    • Candidates must explain that simple distillation separates a solvent from a solution because the solvent has a much lower boiling point
    • Award 1 mark for stating that the baseline in chromatography must be drawn in pencil to prevent it dissolving in the mobile phase

    Examiner Tips

    Expert advice for maximising your marks

    • 💡In 6-mark separation planning questions, explicitly name every piece of apparatus (e.g., 'filter funnel', 'condenser') to secure AO1 marks
    • 💡When justifying a purity check, always reference 'sharp melting point' for pure substances versus 'melting over a range' for mixtures
    • 💡Check your Rf calculation logic: the answer must always be between 0 and 1. If it is >1, you have inverted the fraction

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Stating that a substance is pure because it is 'clean', 'clear', or 'colourless' rather than referencing its chemical composition or melting point
    • Drawing the chromatography start line in ink, which dissolves in the solvent and obscures the results
    • Calculating Rf values greater than 1 by incorrectly dividing the solvent distance by the spot distance
    • Confusing crystallization with evaporation; failing to mention 'heating to saturation point' before cooling

    Key Terminology

    Essential terms to know

    Atomic classification (Element vs Compound vs Mixture)
    Physical separation techniques (Filtration, Crystallisation, Distillation, Chromatography)
    Criteria of purity and melting/boiling point analysis
    Interpretation of chromatograms and Rf values

    Likely Command Words

    How questions on this topic are typically asked

    State
    Describe
    Explain
    Calculate
    Suggest
    Plan

    Practical Links

    Related required practicals

    • •{"code":"PAG C2","title":"Separation techniques","relevance":"Filtration, evaporation, distillation and chromatography methodologies"}

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