Latent Heat

    OCR
    GCSE

    Specific latent heat defines the amount of energy required to change the state of one kilogram of a substance with no change in temperature. This concept distinguishes between the specific latent heat of fusion (solid to liquid) and vaporisation (liquid to gas). During these phase changes, supplied energy increases the internal potential energy of the particles to overcome intermolecular forces, rather than increasing their kinetic energy. Candidates must apply the equation E = mL to calculate energy transfers and interpret heating or cooling curves where temperature plateaus indicate state changes.

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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 a change of state
    • Award 1 mark for linking energy supplied to the breaking of intermolecular bonds or overcoming forces of attraction between particles
    • Award 1 mark for correct substitution of values into E = mL, ensuring mass is converted to match the specific latent heat units
    • Award 1 mark for identifying that internal energy increases due to a rise in potential energy, while kinetic energy remains constant

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the flat section of the graph as a state change; now explain what happens to the energy stores of the particles during this time"
    • "Check your units—you used grams but the constant was in J/kg. Always match your mass unit to the constant provided"
    • "You mentioned bonds breaking, which is good. To secure higher marks, specify that this increases the potential energy of the system"
    • "Excellent calculation. Ensure you explicitly state the final unit (Joules) to guarantee the final mark"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that temperature remains constant during a change of state
    • Award 1 mark for linking energy supplied to the breaking of intermolecular bonds or overcoming forces of attraction between particles
    • Award 1 mark for correct substitution of values into E = mL, ensuring mass is converted to match the specific latent heat units
    • Award 1 mark for identifying that internal energy increases due to a rise in potential energy, while kinetic energy remains constant

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When analyzing heating curves, explicitly state that horizontal sections represent changes of state where potential energy increases but kinetic energy (temperature) is constant
    • 💡Check the units of Specific Latent Heat carefully; if given in J/g, do not convert mass to kg, but if J/kg, conversion is mandatory
    • 💡Memorize the distinction: Fusion refers to the solid-liquid transition; Vaporisation refers to the liquid-gas transition

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing Specific Latent Heat with Specific Heat Capacity when selecting constants or equations
    • Stating that kinetic energy increases during melting or boiling, rather than potential energy
    • Failing to convert mass from grams to kilograms when the specific latent heat value is provided in J/kg

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Specific Latent Heat of Fusion vs Vaporisation
    Internal Energy: Kinetic vs Potential Energy stores
    Heating and Cooling Curves (Temperature Plateaus)
    Quantitative Analysis using E = mL

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Explain
    Describe
    Plot
    Interpret

    Practical Links

    Related required practicals

    • •{"code":"PAG P5","title":"Changes of State","relevance":"Investigation into the temperature changes of ice as it melts and water as it boils"}

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