Specific Latent Heat
Build confidence in Specific Latent Heat for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Specific latent heat is the energy needed to change the state of 1 kg of a substance without changing its temperature, using E = mL.
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Topic overview
Specific latent heat is the amount of energy required to change the state of one kilogram of a substance with no change in temperature.
The equation is energy for a change of state = mass × specific latent heat. There are two versions: specific latent heat of fusion for changing between solid and liquid, and specific latent heat of vaporisation for changing between liquid and gas.
The two must not be confused. Fusion is melting or freezing; vaporisation is boiling or condensing. The specific latent heat of vaporisation is always the larger value, because the particles must be separated completely rather than merely freed from their fixed positions.
Revision notes
The equation
Energy for a change of state = mass × specific latent heat, E = mL.
Mass in kilograms, specific latent heat in J/kg, energy in joules. Note that no temperature appears — the temperature does not change during the process.
Fusion and vaporisation
Specific latent heat of fusion: changing between solid and liquid, so melting or freezing.
Specific latent heat of vaporisation: changing between liquid and gas, so boiling or condensing. Vaporisation always has the larger value.
Why vaporisation is larger
Melting only requires the particles to be freed from their fixed positions; they remain close together.
Boiling requires the particles to be separated completely, overcoming all the forces between them, so far more energy is needed per kilogram.
Key points
- Specific latent heat changes state without changing temperature.
- Energy = mass × specific latent heat.
- Fusion is between solid and liquid.
- Vaporisation is between liquid and gas.
- Vaporisation has the larger value.
- No temperature term appears in the equation.
Worked examples
Example 1
Calculate the energy needed to melt 2 kg of ice. Specific latent heat of fusion of water = 334 000 J/kg. [2 marks]
Model answer
Energy = mass × specific latent heat = 2 × 334 000 (1 mark) = 668 000 J (1 mark).
Example 2
Define specific latent heat. [2 marks]
Model answer
Specific latent heat is the amount of energy required to change the state of one kilogram of a substance (1 mark) with no change in temperature (1 mark).
Example 3
Explain why the specific latent heat of vaporisation is greater than that of fusion. [2 marks]
Model answer
Melting only requires the particles to be freed from their fixed positions, and they remain close together (1 mark). Boiling requires the particles to be separated completely, overcoming all the forces between them, which needs far more energy (1 mark).
Common mistakes
Including a temperature change in the equation.
There is no temperature term — the temperature does not change.
Confusing fusion and vaporisation.
Fusion is solid to liquid; vaporisation is liquid to gas.
Confusing specific latent heat with specific heat capacity.
Latent heat changes state; heat capacity changes temperature.
Using grams instead of kilograms.
Mass must be in kilograms.
Exam tips
- Remember there is no temperature term in this equation.
- Keep fusion and vaporisation clearly distinct.
- Explain the size difference through particle separation.
- Distinguish this from specific heat capacity.
Key terms
- Specific latent heat
- Energy to change the state of 1 kg with no temperature change.
- Fusion
- The change between solid and liquid.
- Vaporisation
- The change between liquid and gas.
- Latent
- Hidden, because no temperature change is observed.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.