Melting and Boiling

    OCR
    GCSE

    Melting and boiling are physical changes of state characterized by the absorption of thermal energy to overcome intermolecular forces, resulting in an increase in internal potential energy while kinetic energy remains constant. Consequently, these transitions occur at fixed temperatures, visible as plateaus on heating curves. Candidates must distinguish between specific latent heat of fusion and vaporisation and apply the equation E = mL to quantify energy transfers.

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

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that temperature remains constant during melting because energy is transferred to potential energy stores to overcome intermolecular forces
    • Credit correct substitution of mass (converted to kg) and specific latent heat into the equation E = mL
    • Award 1 mark for identifying the horizontal section of a temperature-time graph as the specific change of state
    • Candidates must describe the change in particle arrangement from regular lattice (solid) to random arrangement (liquid)

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the melting point, but you must explain *why* the line is flat in terms of energy transfer"
    • "Check your units — you used grams but the specific latent heat constant was given in J/kg"
    • "Good use of the equation. To improve, ensure you distinguish between 'breaking bonds' (chemical) and 'overcoming forces' (physical)"
    • "You described the shape of the graph well; now explain what the particles are doing in each section to secure the AO2 marks"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that temperature remains constant during melting because energy is transferred to potential energy stores to overcome intermolecular forces
    • Credit correct substitution of mass (converted to kg) and specific latent heat into the equation E = mL
    • Award 1 mark for identifying the horizontal section of a temperature-time graph as the specific change of state
    • Candidates must describe the change in particle arrangement from regular lattice (solid) to random arrangement (liquid)

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When explaining the flat section of a heating curve, explicitly state that 'energy is being used to break intermolecular bonds' rather than just 'changing state'
    • 💡Check the units of Specific Latent Heat carefully; if given in J/kg, ensure your mass is converted to kg to avoid a factor of 1000 error
    • 💡For 'Describe the motion' questions, use specific terms: 'vibrating about fixed positions' for solids versus 'moving randomly' for liquids/gases

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Stating that particles themselves expand or melt; candidates must refer to the space between particles increasing or forces being overcome
    • Assuming temperature increases during the change of state; failing to recognize that kinetic energy remains constant while potential energy increases
    • Using the specific heat capacity equation (E = mcΔθ) instead of specific latent heat (E = mL) for state changes

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Internal energy and changes of state
    Specific Latent Heat of Fusion and Vaporisation
    Interpretation of heating and cooling curves
    Particle model mechanics during state transitions

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Explain
    Describe
    State
    Plot

    Practical Links

    Related required practicals

    • •{"code":"PAG P5","title":"Investigation of changes of state","relevance":"Measuring the cooling curve of stearic acid to identify freezing point"}

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