Specific latent heat Revision Notes for AQA GCSE Physics Combined Science

Specific latent heat

What is specific latent heat?

Specific latent heat is the amount of energy needed to change the state of 1 kg of a substance. This energy is needed when materials change between solid, liquid and gas states.

When a substance changes state, its temperature stays the same. The energy goes into breaking or making bonds between particles, not increasing their movement.

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The key concept to understand is that during state changes, all the energy goes into changing the arrangement of particles rather than changing their kinetic energy (temperature).

Types of specific latent heat

There are two main types you need to know:

Specific latent heat of fusion

This is the energy needed when substances change between solid and liquid states.

Melting: solid → liquid (energy added)
Freezing: liquid → solid (energy removed)

For water, the specific latent heat of fusion is 334,000 J/kg. This means you need 334,000 J to melt 1 kg of ice at 0°C.

Specific latent heat of vaporisation

This is the energy needed when substances change between liquid and gas states.

Boiling: liquid → gas (energy added)
Condensing: gas → liquid (energy removed)

For water, the specific latent heat of vaporisation is 2,260,000 J/kg. This means you need 2,260,000 J to turn 1 kg of water into steam at 100°C.

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Notice that vaporisation requires much more energy than fusion - about 7 times more for water. This is because completely separating particles (liquid to gas) requires more energy than just loosening the bonds (solid to liquid).

Formula for calculations

To calculate the energy needed for a state change, use this formula:

$E = m \times L$

Where:

$E$ = energy (in joules, J)
$m$ = mass (in kilogrammes, kg)
$L$ = specific latent heat (in J/kg)

Important facts about temperature

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Critical Concept: Temperature During State Changes

During state changes:

Temperature stays constant
Energy is still being added or removed
The energy breaks bonds between particles instead of making them move faster

This is why ice stays at 0°C while melting, and water stays at 100°C while boiling.

Worked example

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Worked Example: Calculating Energy for Melting Ice

Question: How much energy is needed to melt 25 kg of ice at 0°C?

Solution: Step 1: Identify the formula $E = m \times L$

Step 2: Identify the given values

Mass $(m)$ = 25 kg
Specific latent heat of fusion for water $(L)$ = 334,000 J/kg

Step 3: Substitute and calculate $E = 25 \times 334,000 = 8,350,000 \text{ J}$

Answer: 8,350,000 J of energy is needed

Summary

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Key Points to Remember:

Specific latent heat is energy needed to change 1 kg of substance between states
Fusion = solid ↔ liquid changes (334,000 J/kg for water)
Vaporisation = liquid ↔ gas changes (2,260,000 J/kg for water)
Use formula: $E = m \times L$
Temperature stays constant during state changes

Specific latent heat (AQA GCSE Physics Combined Science): Revision Notes