Specific latent heat: the energy needed to change the state of a substance, the latent heat equation, and the difference between latent heat of fusion and vaporisation.

A focused answer to AQA GCSE Physics 4.3.2, covering specific latent heat as the energy needed to change the state of one kilogram of a substance, the latent heat equation, and the difference between the specific latent heat of fusion and of vaporisation.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
What specific latent heat means
The latent heat equation
Fusion and vaporisation
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What this dot point is asking

AQA wants you to define specific latent heat, use the latent heat equation, and distinguish the specific latent heat of fusion from the specific latent heat of vaporisation. This is part of topic 4.3.2 of the AQA GCSE Physics (8463) specification.

What specific latent heat means

The energy goes into changing the particles' potential energy (breaking or forming forces between them), which is why the temperature stays constant during the change of state. The word "latent" means hidden, because the energy supplied is not seen as a temperature rise on the thermometer; it is stored in the changed arrangement of the particles instead. When the change is reversed (for example when a gas condenses or a liquid freezes), exactly the same amount of energy is released to the surroundings, which is why steam at $100\,^{\circ}C$ causes much worse scalds than water at $100\,^{\circ}C$ : as the steam condenses on the skin it releases its large latent heat of vaporisation.

It is important not to mix this equation up with the specific heat capacity equation. Specific heat capacity ( $\Delta E = mc\Delta\theta$ ) deals with changing the temperature of a substance within one state, while specific latent heat ( $E = mL$ ) deals with changing the state at constant temperature. A full heating problem (for example ice below zero turned into steam) is solved in stages, using the heat capacity equation for each sloped temperature rise and the latent heat equation for each change of state.

The latent heat equation

Fusion and vaporisation

For water the figures show the contrast clearly: the specific latent heat of fusion is about $334{,}000\,J/kg$ , while the specific latent heat of vaporisation is about $2{,}260{,}000\,J/kg$ , nearly seven times larger. Melting only needs enough energy to loosen the particles so they can slide past one another, but vaporising needs enough energy to pull every particle right away from its neighbours so they can move independently as a gas, which is why so much more energy is required. This large latent heat of vaporisation is why sweating cools the body effectively (the evaporating sweat carries away a lot of energy) and why steam heating systems can transfer a great deal of energy as the steam condenses.

Try this

Q1. Define the specific latent heat of vaporisation. [2 marks]

Cue. The energy needed to change $1\,kg$ of a substance from liquid to gas with no change in temperature.

Q2. Calculate the energy needed to boil away $0.5\,kg$ of water, given the specific latent heat of vaporisation is $2260000\,J/kg$ . [2 marks]

Cue. $E = mL = 0.5 \times 2260000 = 1130000\,J = 1.13\,MJ$ .

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20205 marksA heater supplies energy to melt

0.40\,\text{kg}

of ice at its melting point. The specific latent heat of fusion of water is

334{,}000\,\text{J/kg}

. Calculate the energy needed to melt all the ice, and explain why this energy does not raise the temperature of the ice while it is melting.

Show worked answer →

Use the latent heat equation $E = mL = 0.40 \times 334{,}000 = 133{,}600\,\text{J}$ , which is about $1.34 \times 10^{5}\,\text{J}$ or $134\,\text{kJ}$ (3 marks). The energy supplied does not raise the temperature because, during melting, it is used to break the forces holding the particles in the solid structure, increasing the potential energy of the particles rather than their kinetic energy (1 mark); since temperature depends on the average kinetic energy of the particles, and that is unchanged, the temperature stays constant at the melting point until all the ice has melted (1 mark). Markers reward the correct calculation using $E = mL$ and the explanation linking constant temperature to potential rather than kinetic energy.

AQA 20214 marksDistinguish between the specific latent heat of fusion and the specific latent heat of vaporisation, and explain why, for the same substance, the specific latent heat of vaporisation is larger.

Show worked answer →

The specific latent heat of fusion is the energy needed to change $1\,\text{kg}$ of a substance between the solid and liquid states (melting or freezing) with no temperature change (1 mark). The specific latent heat of vaporisation is the energy needed to change $1\,\text{kg}$ between the liquid and gas states (boiling or condensing) with no temperature change (1 mark). The latent heat of vaporisation is larger because turning a liquid into a gas requires the forces between the particles to be broken completely, so the particles can move far apart and become independent (1 mark), whereas melting only loosens the particles enough to flow past one another, which needs less energy (1 mark). Markers reward both definitions and the reasoning that vaporisation fully separates the particles while fusion only partly frees them.

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Sources & how we know this

AQA GCSE Physics (8463) specification — AQA (2016)