Purity and Formulations

    OCR
    GCSE

    Chemically pure substances consist of a single element or compound, distinct from the colloquial definition implying natural origin. Purity is quantitatively assessed through physical properties, specifically melting and boiling points; pure substances exhibit sharp, distinct transition temperatures, whereas impurities lower melting points and broaden the melting range. Formulations are mixtures designed as useful products—such as fuels, medicines, and alloys—where components are combined in precise ratios to achieve specific properties. Chromatography is the primary analytical technique used to distinguish pure substances from mixtures and to identify components based on their differential affinity for the mobile and stationary phases.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
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    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 a single element or compound not mixed with any other substance
    • Credit responses that state pure substances have a sharp, specific melting point, whereas mixtures melt over a temperature range
    • Award 1 mark for defining a formulation as a mixture designed as a useful product with specific proportions
    • For chromatography, award marks for calculating Rf values using the formula: distance moved by substance ÷ distance moved by solvent front
    • Credit the explanation that the baseline is drawn in pencil because graphite is insoluble in the solvent and will not interfere with the chromatogram

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly calculated the Rf value, but ensure you measure from the baseline, not the paper edge"
    • "Good definition of a mixture, but you must specify that a formulation has 'specific proportions' for the intended function"
    • "You identified the substance as impure; now explain how the melting point range confirms this"
    • "Excellent use of the term 'mobile phase' and 'stationary phase'—ensure you link these to the solubility of the components"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for defining a pure substance as consisting of a single element or compound not mixed with any other substance
    • Credit responses that state pure substances have a sharp, specific melting point, whereas mixtures melt over a temperature range
    • Award 1 mark for defining a formulation as a mixture designed as a useful product with specific proportions
    • For chromatography, award marks for calculating Rf values using the formula: distance moved by substance ÷ distance moved by solvent front
    • Credit the explanation that the baseline is drawn in pencil because graphite is insoluble in the solvent and will not interfere with the chromatogram

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When asked to justify purity using data, explicitly reference the 'sharpness' of the melting point, not just the value
    • 💡In Rf calculations, always check your answer is less than 1.0; if it is greater, you have inverted the ratio
    • 💡Memorise specific examples of formulations (e.g., alloys, fertilisers, cleaning agents) as OCR often requires context-based application
    • 💡Use the terms 'mobile phase' and 'stationary phase' correctly when explaining how separation occurs based on solubility

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing the everyday marketing definition of 'pure' (e.g., natural fruit juice) with the chemical definition (single substance)
    • Measuring the distance moved by the solvent from the bottom of the paper rather than from the pencil baseline
    • Stating that impurities increase the melting point; candidates must recognise impurities lower the melting point and broaden the range
    • Drawing the start line in ink, which dissolves in the mobile phase and ruins the separation

    Key Terminology

    Essential terms to know

    Chemical purity vs. everyday purity
    Melting and boiling point analysis
    Design and function of formulations
    Chromatographic separation and Rf values

    Likely Command Words

    How questions on this topic are typically asked

    Define
    Explain
    Calculate
    Analyse
    Suggest
    Compare

    Practical Links

    Related required practicals

    • •{"code":"PAG C4","title":"Separation techniques (Chromatography)","relevance":"Analysis of food dyes or inks to determine purity and composition"}

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