AQA · A-Level · Physics

    Course version: 7408 · Version from Sept 2016

    For a change of state Q = ml where l is the specific latent heat.

    Full specification wording

    Thermal energy transfer (A-level only): For a change of state Q = ml where l is the specific latent heat.

    During a change of state the temperature stays constant while energy is transferred, so specific heat capacity cannot be used.

    Checked against the AQA 7408 specification (opens in a new tab) on .

    • Point 7 of 7
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    During a change of state the temperature stays constant while energy is transferred, so specific heat capacity cannot be used. Instead Q = ml, where Q is the energy needed to change the state of mass m and l is the specific latent heat: the energy per unit mass for that change. Melting and boiling use the latent heat of fusion and of vaporisation respectively; condensation and freezing release the same energy per unit mass. For example, melting 0.50 kg of ice needs Q = 0.50 × 3.34 × 10⁵ = 1.67 × 10⁵ J. Rearranged, l = Q ÷ m, so l has units J kg⁻¹. In a mixture problem, energy lost by cooling equals energy gained by melting, so equate mcΔθ with ml.

    What earns the marks

    • States that temperature remains constant during a change of state, so Q = mcΔθ cannot be applied and Q = ml is used instead.
    • Defines specific latent heat as the energy required per unit mass to change the state of a substance at constant temperature.
    • Applies Q = ml with consistent units, converting grams to kilograms and kJ to J before substituting.
    • Distinguishes latent heat of fusion (solid to liquid) from latent heat of vaporisation (liquid to gas), and recognises that the reverse changes release the same energy per unit mass.
    • Rearranges to l = Q ÷ m and identifies the unit J kg⁻¹, or uses simultaneous energy balance in mixtures.
    • Interprets a heating graph: sloping sections use mcΔθ, flat sections use ml, and the longer flat section at boiling reflects the larger latent heat of vaporisation.

    Common mistakes and how to put them right

    • Using Q = mcΔθ during melting or boiling.

      Put it right by checking that the temperature is constant and switching to Q = ml.

    • Forgetting to convert grams to kilograms or kJ to J.

      Put it right by converting all masses to kg and energies to J before substituting.

    • Assuming the energy supplied always equals ml in an experiment.

      Put it right by accounting for energy lost to the surroundings, for example by using a lid or lagging and treating the value as a minimum.

    Exam tips

    • Underline the phrase change of state in the question; it signals that Q = ml is expected rather than Q = mcΔθ.
    • Show the substitution with units before evaluating, so a unit slip is visible and can still earn method credit.
    • When a question gives a rate in W, multiply power by time to find Q before dividing by m to obtain l.

    Try the lesson

    The start of the Thermal energy transfer (A-level only): For a change of state Q = ml where l is the specific latent heat. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A mass of 0.40 kg of ice melts completely at 0 °C. The specific latent heat of fusion of ice is 3.3 × 10⁵ J kg⁻¹. Calculate the energy transferred to the ice during melting.

    Listen

    During a change of state the temperature stays constant while energy is transferred, so specific heat capacity cannot be used. Instead Q = ml, where Q is the energy needed to change the state of mass m and l is the specific latent heat: the energy per unit mass for that change. Melting and boiling use the latent heat of fusion and of vaporisation respectively; condensation and freezing release the same energy per unit mass. For example, melting 0.50 kg of ice needs Q = 0.50 × 3.34 × 10⁵ = 1.67 × 10⁵ J. Rearranged, l = Q ÷ m, so l has units J kg⁻¹. In a mixture problem, energy lost by cooling equals energy gained by melting, so equate mcΔθ with ml. A strong answer earns marks for these points. States that temperature remains constant during a change of state, so Q = mcΔθ cannot be applied and Q = ml is used instead. Defines specific latent heat as the energy required per unit mass to change the state of a substance at constant temperature. Applies Q = ml with consistent units, converting grams to kilograms and kJ to J before substituting. Distinguishes latent heat of fusion (solid to liquid) from latent heat of vaporisation (liquid to gas), and recognises that the reverse changes release the same energy per unit mass. A common slip: Using Q = mcΔθ during melting or boiling. Instead, checking that the temperature is constant and switching to Q = ml. Exam tip: Underline the phrase change of state in the question; it signals that Q = ml is expected rather than Q = mcΔθ.

    See it

    A flowchart starting from a node labelled Change of state, which leads to Temperature stays constant. From there two branches: Q = mcΔθ cannot be used, and Use Q = ml. From Use Q = ml a node leads to l = specific latent heat, which splits into Energy per unit mass and Unit J kg⁻¹. Use Q = ml also leads to two further nodes: Melting and boiling: fusion and vaporisation, and Condensation and freezing: release same energy per unit mass.
    It shows that during a change of state the temperature stays constant, so Q = ml is used instead of Q = mcΔθ, with l defined as the energy per unit mass.

    Key words

    Say what each word means before you turn the card.

    Specific latent heat

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    Latent heat of fusion

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    Latent heat of vaporisation

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    Q = ml

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    See every definition
    Specific latent heat
    The energy required per unit mass to change the state of a substance at constant temperature.
    Latent heat of fusion
    The latent heat used for melting, the change from solid to liquid.
    Latent heat of vaporisation
    The latent heat used for boiling, the change from liquid to gas.
    Q = ml
    The equation used instead of Q = mcΔθ during a change of state, where Q is the energy needed to change the state of mass m.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A flowchart starting from a node labelled Change of state, which leads to Temperature stays constant. From there two branches: Q = mcΔθ cannot be used, and Use Q = ml. From Use Q = ml a node leads to l = specific latent heat, which splits into Energy per unit mass and Unit J kg⁻¹. Use Q = ml also leads to two further nodes: Melting and boiling: fusion and vaporisation, and Condensation and freezing: release same energy per unit mass.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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