Particle motion in gases

    AQA
    GCSE

    The kinetic particle model defines gases as molecules in constant, random motion, where temperature is directly proportional to the average kinetic energy of the particles. Gas pressure arises from the frequency and force of particle collisions against container walls, creating a net force per unit area. Candidates must quantify relationships between pressure, volume, and temperature using the Kelvin scale and apply Boyle's Law (pV = constant) to fixed masses of gas at constant temperature. Higher-tier analysis requires explaining how doing work on a gas increases its internal energy and temperature.

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    Objectives
    3
    Exam Tips
    3
    Pitfalls
    5
    Key Terms
    4
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that gas particles move in random directions at a range of speeds
    • Credit responses that link an increase in temperature to an increase in the average kinetic energy of the particles
    • Award marks for explaining pressure: particles collide with walls, exerting a force at right angles (normal) to the surface
    • For Higher Tier, award credit for explaining that doing work on a gas (compression) increases its internal energy and causes a temperature rise

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that gas particles move in random directions at a range of speeds
    • Credit responses that link an increase in temperature to an increase in the average kinetic energy of the particles
    • Award marks for explaining pressure: particles collide with walls, exerting a force at right angles (normal) to the surface
    • For Higher Tier, award credit for explaining that doing work on a gas (compression) increases its internal energy and causes a temperature rise

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When explaining pressure changes, use the 'Collisions-Force-Area' chain: more frequent collisions → greater total force → higher pressure
    • 💡Memorise that 'temperature' is a measure of the 'average kinetic energy' of particles; these exact phrases are often required for AO1 marks
    • 💡For Boyle's Law calculations (pV = constant), ensure you identify the 'before' and 'after' states clearly before substituting values

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Attributing gas pressure to collisions between particles rather than collisions with the container walls
    • Describing gas particles as 'vibrating' (a property of solids) instead of moving with random translational motion
    • Failing to convert units (e.g., kPa to Pa or cm³ to m³) when using the equation pV = constant, or assuming linear relationships where inverse proportionality applies

    Key Terminology

    Essential terms to know

    Likely Command Words

    How questions on this topic are typically asked

    State
    Explain
    Calculate
    Describe
    Suggest

    Practical Links

    Related required practicals

    • •{"code":"Required Practical 5","title":"Determination of density","relevance":"Foundational concept for the Particle Model of Matter module, underpinning mass/volume relationships essential for understanding gas behaviour."}

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