Define and use specific latent heat
Calculate energy transferred during a change of state and distinguish fusion from vaporisation.
Section 14 · Temperature
A Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 14. These original explanations and calculations are study aids; full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use temperature differences, proportional reasoning, energy, mass and SI units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens to temperature during a pure substance's phase change at fixed pressure?
What you will learn
- Define specific latent heat
- Use Q = mL for a change of state
- Distinguish specific latent heat of fusion from vaporisation
Latent heat changes state without changing temperature
The specific latent heat L is the energy required per unit mass to change a substance's state without a temperature change during that transition. Its unit is J kg⁻¹. For mass m, the energy transferred is Q = mL.
During a state change, energy changes the arrangement or separation of particles rather than raising or lowering the temperature. The temperature remains constant while the phase change is taking place at a fixed pressure.
Fusion and vaporisation are different transitions
Specific latent heat of fusion applies to melting or freezing between solid and liquid. Specific latent heat of vaporisation applies to boiling, evaporation or condensation between liquid and gas.
The energy magnitude for a given mass depends on which transition occurs. Identify the initial and final states before selecting the relevant L, then use mass in kilograms.
Pause and explain
Which transition uses specific latent heat of vaporisation?
Worked example
How much energy is required to melt 0.080 kg of a substance with specific latent heat of fusion 3.34 × 10⁵ J kg⁻¹?
Show the worked solution
- Identify this as a fusion change.
- Use Q = mL.
- Multiply 0.080 by 3.34 × 10⁵ J kg⁻¹.
Answer Q = 2.67 × 10⁴ J.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Calculate the energy to melt 0.12 kg of ice using Lf = 3.34 × 10⁵ J kg⁻¹.
Give me a hint
Use Q = mL and convert no units because mass is already in kilograms.
Compare my reasoning
- This is fusion, so use Lf.
- Q = 0.12 × 3.34 × 10⁵.
- Give joules.
Q = 4.01 × 10⁴ J.
Look for these in your work
- Fusion identified
- Mass and L multiplied
- Energy unit stated
A 0.020 kg liquid sample vaporises; Lv = 2.26 × 10⁶ J kg⁻¹. Find the energy transferred.
Give me a hint
Choose latent heat of vaporisation for the liquid-to-gas transition.
Compare my reasoning
- Q = mLv.
- Q = 0.020 × 2.26 × 10⁶.
- Evaluate in joules.
Q = 4.52 × 10⁴ J.
Look for these in your work
- Vaporisation selected
- Mass in kilograms
- Energy calculated
A learner uses Q = mcΔT to estimate energy while a substance is melting at constant temperature. Explain the mistake and identify the appropriate model.
Give me a hint
During melting ΔT is zero even though energy is transferred; identify what changes instead.
Compare my reasoning
- The material changes from solid to liquid while its temperature stays constant.
- The mcΔT model calculates energy associated with a temperature change.
- Use Q = mLf for the fusion transition.
Use Q = mLf: energy is transferred for the change of state even though the temperature remains constant.
Look for these in your work
- Transition identified
- Temperature condition explained
- Latent-heat model selected
Common mistakes
- Using Q = mcΔT during the phase change itself
- Confusing fusion with vaporisation
- Using grams with a latent heat stated per kilogram
What can you explain now?
What happens to temperature during a pure substance's phase change at fixed pressure?
Compare with the explanation
It remains constant during the transition
Transferred energy changes state rather than temperature during the change.
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Make it stick
How does latent heat differ from specific heat capacity, and which equation applies to each?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.