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    Specific Latent Heat Calculator (E = mL)

    GCSE PhysicsAQAEdexcelOCRFoundation & Higher

    Find the energy for a change of state, the mass or the specific latent heat using E = m × L. Works for melting, freezing, boiling and condensing.

    Quick answer

    Energy = mass × specific latent heat. Use mass in kg and L in J/kg to get joules. For example, melting 0.50 kg of ice with L = 334,000 J/kg takes 0.50 × 334,000 = 167,000 J. The temperature doesn't change during a change of state.

    By Engr. Mehmood Kakar · Updated

    E = m × L
    Find:

    Use the value given in the question. Melting (fusion) and boiling (vaporisation) have different values.

    Simulation

    Energy in, but no temperature rise

    Choose melting or boiling and the mass of water. The heating curve shows the flat section where energy changes the state, not the temperature.

    0.5 kg

    During a change of state the temperature stays constant: the energy breaks bonds between particles instead of making them move faster. Boiling water needs about 7 times more energy than melting the same mass of ice.

    Put these numbers in the calculator above

    How to use E = mL

    When a substance melts, freezes, boils or condenses, energy is transferred but its temperature stays the same. Specific latent heat is the energy needed to change the state of 1 kg of a substance with no change in temperature:

    energy for a change of state = mass × specific latent heat

    • E is energy in joules (J)
    • m is mass in kilograms (kg)
    • L is specific latent heat in joules per kilogram (J/kg)

    There are two values for each substance. The specific latent heat of fusion is for melting or freezing. The specific latent heat of vaporisation is for boiling or condensing. Exam questions give you the value to use. For water, questions often give 334,000 J/kg for fusion and 2,260,000 J/kg for vaporisation.

    Melting 0.50 kg of ice at 0 °C takes 0.50 × 334,000 = 167,000 J. Notice that the temperature does not appear anywhere in the equation. If the temperature changes, use ΔE = mcΔθ instead.

    Common mistakes that cost marks

    1. Using the wrong value of L. Fusion is for melting and freezing; vaporisation is for boiling and condensing.
    2. Using ΔE = mcΔθ during a change of state. While ice is melting its temperature stays at 0 °C, so use E = mL.
    3. Not converting grams. 200 g is 0.2 kg.
    4. Not converting kJ/kg. 334 kJ/kg is 334,000 J/kg.

    Practice questions

    Try each one on paper first, then reveal the worked answer.

    Q1Calculate the energy needed to melt 0.50 kg of ice at 0 °C. Specific latent heat of fusion of water = 334,000 J/kg.[2 marks]
    1. E = m × L
    2. E = 0.50 × 334,000
    3. E = 167,000 J
    Q2Calculate the energy needed to boil away 200 g of water at 100 °C. Specific latent heat of vaporisation of water = 2,260,000 J/kg.[3 marks]
    1. E = m × L
    2. Convert: m = 200 ÷ 1000 = 0.20 kg
    3. E = 0.20 × 2,260,000
    4. E = 452,000 J
    Q3A kettle transfers 1.13 MJ of energy to boiling water. Specific latent heat of vaporisation of water = 2.26 MJ/kg. Calculate the mass of water turned to steam.[3 marks]
    1. Rearrange: m = E ÷ L
    2. m = 1,130,000 ÷ 2,260,000
    3. m = 0.50 kg

    Endless practice

    A new question every time, marked instantly. Work it out on paper, then type your answer.

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    Frequently asked questions

    What is the equation for specific latent heat?

    Energy for a change of state = mass × specific latent heat (E = mL). Energy is in joules (J), mass in kg and specific latent heat in J/kg.

    What is the difference between latent heat of fusion and vaporisation?

    Fusion is the energy to melt (or freeze) 1 kg of a substance. Vaporisation is the energy to boil (or condense) 1 kg. Vaporisation is much larger for water.

    Why does temperature stay constant during a change of state?

    The energy transferred breaks or forms bonds between particles, changing the internal energy without increasing the particles’ kinetic energy. So the temperature stays the same until the change of state is complete.

    When do I use E = mL instead of ΔE = mcΔθ?

    Use E = mL when the state changes at a constant temperature. Use ΔE = mcΔθ when the temperature changes and the state does not.