OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Apply the relationship between specific latent heat and mass to calculate…
Full specification wording
Changes of state: apply the relationship between specific latent heat and mass to calculate the energy change involved in a change of state
Specific latent heat is the energy needed to change the state of 1 kg of a substance without changing its temperature.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 8 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Specific latent heat is the energy needed to change the state of 1 kg of a substance without changing its temperature. The relationship is E = m × L, where E is the energy change in joules (J), m is the mass in kilograms (kg) and L is the specific latent heat in joules per kilogram (J/kg). To apply it, identify the mass and the specific latent heat for the change happening (melting, freezing, boiling or condensing), then multiply. For example, melting 0.50 kg of ice with a specific latent heat of fusion of 3.34 × 10⁵ J/kg needs E = 0.50 × 3.34 × 10⁵ = 1.67 × 10⁵ J. Rearranged forms give m = E ÷ L and L = E ÷ m. Keep mass in kg and energy in J before calculating.
What earns the marks
- Multiplies the mass in kilograms by the specific latent heat in joules per kilogram to obtain the energy in joules.
- Selects the specific latent heat appropriate to the stated change of state rather than a value for a different change.
- Converts a mass given in grams to kilograms by dividing by 1000 before substituting into the relationship.
- Rearranges the relationship correctly to find mass or specific latent heat when the energy change is given.
- Keeps units consistent, expressing energy in joules and specific latent heat in joules per kilogram.
Common mistakes and how to put them right
Uses the specific heat capacity instead of the specific latent heat.
Put it right: Select the latent heat value because the temperature stays constant during a change of state.
Forgets to convert grams to kilograms.
Put it right: Divide the mass by 1000 so it is in kg before multiplying.
Adds the temperature change into the calculation.
Put it right: Omit any temperature term because latent heat applies at constant temperature.
Mixes up the latent heat of fusion and the latent heat of vaporisation.
Put it right: Match the value to the change of state described.
Exam tips
- Write down E = m × L and label each known quantity with its unit before substituting numbers.
- Check whether the mass is in grams and convert to kilograms first, since the specific latent heat is per kilogram.
- Sanity-check the answer: a few tenths of a kilogram typically needs tens or hundreds of thousands of joules to change state.
Try the lesson
The start of the Changes of state: apply the relationship between specific latent heat and mass to calculate the energy change involved in a change of state lesson, made from checked teaching notes for this specification point.
Have a go first
A block of ice melts at 0 °C. The specific latent heat of fusion of water is 334 000 J/kg and the mass of ice that melts is 0.50 kg. Calculate the energy needed to melt the ice.
Listen
Specific latent heat is the energy needed to change the state of 1 kg of a substance without changing its temperature. The relationship is E = m × L, where E is the energy change in joules (J), m is the mass in kilograms (kg) and L is the specific latent heat in joules per kilogram (J/kg). To apply it, identify the mass and the specific latent heat for the change happening (melting, freezing, boiling or condensing), then multiply. For example, melting 0.50 kg of ice with a specific latent heat of fusion of 3.34 × 10⁵ J/kg needs E = 0.50 × 3.34 × 10⁵ = 1.67 × 10⁵ J. Rearranged forms give m = E ÷ L and L = E ÷ m. Keep mass in kg and energy in J before calculating. A strong answer earns marks for these points. Multiplies the mass in kilograms by the specific latent heat in joules per kilogram to obtain the energy in joules. Selects the specific latent heat appropriate to the stated change of state rather than a value for a different change. Converts a mass given in grams to kilograms by dividing by 1000 before substituting into the relationship. Rearranges the relationship correctly to find mass or specific latent heat when the energy change is given. Exam tip: Write down E = m × L and label each known quantity with its unit before substituting numbers.
See it
Key words
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Specific latent heat
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Energy, mass and specific latent heat relationship
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Rearranging equations
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Unit consistency in energy calculations
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See every definition
- Specific latent heat
- The energy needed to change the state of 1 kg of a substance without changing its temperature.
- Energy, mass and specific latent heat relationship
- E = m × L, where E is the energy change in joules, m is the mass in kilograms and L is the specific latent heat in joules per kilogram.
- Rearranging equations
- The rearranged forms give m = E ÷ L and L = E ÷ m.
- Unit consistency in energy calculations
- Keep mass in kg and energy in J before calculating.
Label the diagram
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