Absolute Zero

    OCR
    GCSE

    Absolute zero represents the theoretical lower limit of the thermodynamic temperature scale, defined as -273°C or 0 Kelvin, where particles possess minimum internal energy and molecular motion effectively ceases. Candidates must demonstrate understanding of the Kelvin scale, specifically that temperature in Kelvin is directly proportional to the average kinetic energy of particles. Analysis of pressure-temperature or volume-temperature graphs is required to identify absolute zero via linear extrapolation to the x-intercept. Proficiency in converting between degrees Celsius and Kelvin is essential for correctly applying gas laws.

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    Objectives
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    Exam Tips
    4
    Pitfalls
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    Key Terms
    4
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that absolute zero is -273 °C or 0 Kelvin
    • Credit responses that explicitly link absolute zero to the point where particles have zero kinetic energy or stop moving
    • Award 1 mark for the correct conversion application: Temperature in K = Temperature in °C + 273
    • For graph questions, credit the extrapolation of a pressure-temperature line (in °C) back to the x-axis intercept at -273 °C

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified -273 °C, but check your conversion direction—did you add or subtract 273?"
    • "Good definition of absolute zero. Now explain what happens to the pressure of a gas at this temperature in terms of collisions."
    • "Remember, the graph only passes through the origin if the temperature axis is in Kelvin."
    • "You used Celsius in the ratio calculation; always convert to Kelvin for gas laws to ensure proportionality holds."

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that absolute zero is -273 °C or 0 Kelvin
    • Credit responses that explicitly link absolute zero to the point where particles have zero kinetic energy or stop moving
    • Award 1 mark for the correct conversion application: Temperature in K = Temperature in °C + 273
    • For graph questions, credit the extrapolation of a pressure-temperature line (in °C) back to the x-axis intercept at -273 °C

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When solving gas law problems, write 'T(K) = T(°C) + 273' at the top of your working to prevent unit errors
    • 💡In 'Explain' questions, explicitly mention 'average kinetic energy' decreases as temperature decreases
    • 💡Check graph axes carefully: if the x-axis is Celsius, the line intersects at -273; if Kelvin, it goes through the origin

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Using Celsius values directly in pressure-temperature ratio calculations instead of converting to Kelvin first
    • Stating that particles stop moving at 0 °C (freezing point of water) rather than at absolute zero (-273 °C)
    • Incorrectly subtracting 273 when converting from Celsius to Kelvin (e.g., calculating 20 °C as -253 K)
    • Drawing the line through the origin (0,0) on a Pressure vs. Temperature (°C) graph

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    The Kelvin vs. Celsius temperature scales
    Particle motion and kinetic energy at 0 K
    Extrapolation of gas law graphs
    Linear relationship between Pressure and Temperature (Kelvin)

    Likely Command Words

    How questions on this topic are typically asked

    State
    Calculate
    Explain
    Describe
    Plot

    Practical Links

    Related required practicals

    • •{"code":"P1.2 Investigation","title":"Investigating the relationship between pressure and temperature of a gas","relevance":"Data from this practical is extrapolated to find the value of absolute zero"}

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