Changes of state

    AQA
    GCSE

    Changes of state are physical changes where mass is conserved, distinct from chemical changes, involving the transfer of energy to or from a system. This process alters the internal energy—specifically the potential energy store of the particles—while the kinetic energy (temperature) remains constant during the transition itself. Candidates must analyze heating and cooling curves to identify melting and boiling points and apply the specific latent heat equation to calculate energy transfers. A robust understanding of the particle model is essential to explain density changes and the arrangement of particles in solids, liquids, and gases.

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    Objectives
    3
    Exam Tips
    4
    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 mass is conserved during a change of state because the number of particles remains unchanged
    • Award 1 mark for identifying that changes of state are physical changes and are therefore reversible
    • Award 1 mark for explaining that during a change of state, energy supplied increases the internal potential energy to break intermolecular bonds rather than increasing kinetic energy
    • Award 1 mark for correct substitution into E = mL to calculate specific latent heat, including valid unit conversion

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that mass is conserved during a change of state because the number of particles remains unchanged
    • Award 1 mark for identifying that changes of state are physical changes and are therefore reversible
    • Award 1 mark for explaining that during a change of state, energy supplied increases the internal potential energy to break intermolecular bonds rather than increasing kinetic energy
    • Award 1 mark for correct substitution into E = mL to calculate specific latent heat, including valid unit conversion

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When analyzing heating curves, explicitly state that 'temperature remains constant' during the flat sections because energy is being used to overcome forces of attraction
    • 💡Distinguish clearly between Specific Latent Heat of Fusion (solid-liquid) and Vaporisation (liquid-gas) when selecting constants for calculations
    • 💡For 6-mark questions on density, describe the use of a displacement can (Eureka can) for irregular solids, ensuring you mention measuring the volume of displaced water

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Stating that the temperature of a substance increases while it is boiling or melting
    • Confusing Specific Latent Heat (energy for state change) with Specific Heat Capacity (energy for temperature change)
    • Describing individual particles as expanding or contracting, rather than the spacing between them changing
    • Failing to convert mass from grams to kilograms when the Specific Latent Heat is given in J/kg

    Key Terminology

    Essential terms to know

    Likely Command Words

    How questions on this topic are typically asked

    State
    Explain
    Calculate
    Determine
    Compare
    Sketch

    Practical Links

    Related required practicals

    • •{"code":"Required Practical 5","title":"Density","relevance":"Determination of density for regular and irregular solids and liquids, often linked to particle arrangement in different states"}

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