OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(c) specific latent heat of fusion and specific latent heat of…
Full specification wording
Thermal properties of materials: (c) specific latent heat of fusion and specific latent heat of vaporisation; E = m l
Specific latent heat of fusion is the energy needed to change unit mass of a substance from solid to liquid at its melting point without a change in temperature.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 2 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Specific latent heat of fusion is the energy needed to change unit mass of a substance from solid to liquid at its melting point without a change in temperature. Specific latent heat of vaporisation is the energy needed to change unit mass from liquid to vapour at its boiling point without a change in temperature. The energy transferred is E = m l, where m is the mass and l is the specific latent heat.
For example: Melting 0.50 kg of ice with a specific latent heat of fusion of 3.34 × 10⁵ J kg⁻¹ requires E = 0.50 × 3.34 × 10⁵ = 1.67 × 10⁵ J. During a change of state the temperature stays constant, so the energy supplied changes the arrangement of particles rather than their kinetic energy.
What earns the marks
- Defines specific latent heat of fusion as the energy per unit mass to change from solid to liquid at constant temperature.
- Defines specific latent heat of vaporisation as the energy per unit mass to change from liquid to vapour at constant temperature.
- States and applies E = m l, using the correct specific latent heat for the change of state.
- Recognises that temperature remains constant during a change of state, so the energy supplied does not raise the temperature.
- Uses correct units, J kg⁻¹ for specific latent heat and J for energy, and converts mass to kg where necessary.
Common mistakes and how to put them right
Using the specific heat capacity equation instead of the latent heat equation during a change of state.
Put it right: Use E = m l when the temperature is constant.
Mixing up fusion and vaporisation.
Put it right: Remember that fusion is solid to liquid and vaporisation is liquid to gas.
Forgetting to convert grams to kilograms.
Put it right: Divide the mass in grams by 1000 before substituting.
Assuming the temperature rises while the substance is melting or boiling.
Put it right: State that the temperature stays constant during the change of state.
Exam tips
- Check whether the question involves a temperature change or a change of state before choosing between E = m c Δθ and E = m l.
- Write down the value and unit of the specific latent heat you are using, and make sure it matches fusion or vaporisation.
- Convert mass to kilograms and keep powers of ten consistent when substituting.
Try the lesson
The start of the Thermal properties of materials: (c) specific latent heat of fusion and specific latent heat of vaporisation; E = m l lesson, made from checked teaching notes for this specification point.
Listen
Specific latent heat of fusion is the energy needed to change unit mass of a substance from solid to liquid at its melting point without a change in temperature. Specific latent heat of vaporisation is the energy needed to change unit mass from liquid to vapour at its boiling point without a change in temperature. The energy transferred is E = m l, where m is the mass and l is the specific latent heat. For example, melting 0.50 kg of ice with a specific latent heat of fusion of 3.34 × 10⁵ J kg⁻¹ requires E = 0.50 × 3.34 × 10⁵ = 1.67 × 10⁵ J. During a change of state the temperature stays constant, so the energy supplied changes the arrangement of particles rather than their kinetic energy. A strong answer earns marks for these points. Defines specific latent heat of fusion as the energy per unit mass to change from solid to liquid at constant temperature. Defines specific latent heat of vaporisation as the energy per unit mass to change from liquid to vapour at constant temperature. States and applies E = m l, using the correct specific latent heat for the change of state. Recognises that temperature remains constant during a change of state, so the energy supplied does not raise the temperature. A common slip: Using the specific heat capacity equation instead of the latent heat equation during a change of state. Instead, use E = m l when the temperature is constant. Exam tip: Check whether the question involves a temperature change or a change of state before choosing between E = m c Δθ and E = m l.
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Key words
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Specific latent heat of fusion
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Specific latent heat of vaporisation
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Energy transfer and the equation E = m l
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Changes of state and constant temperature
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See every definition
- Specific latent heat of fusion
- The energy needed to change unit mass of a substance from solid to liquid at its melting point without a change in temperature.
- Specific latent heat of vaporisation
- The energy needed to change unit mass from liquid to vapour at its boiling point without a change in temperature.
- Energy transfer and the equation E = m l
- The energy transferred is E = m l, where m is the mass and l is the specific latent heat.
- Changes of state and constant temperature
- During a change of state the temperature stays constant, so the energy supplied changes the arrangement of particles rather than their kinetic energy.
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