Specific latent heat Revision Notes for AQA GCSE Physics

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 material. This happens when substances change from one state of matter to another, like when ice melts into water or water boils into steam.

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The key thing to remember is that temperature stays the same during these changes. All the energy goes into breaking or forming bonds between particles, not heating them up.

Two types of specific latent heat

There are two main types you need to know:

Specific latent heat of fusion

This happens when materials change between solid and liquid states. Common examples include ice melting into water, or water freezing into ice.

For water, the specific latent heat of fusion is 334,000 J/kg.

Specific latent heat of vaporisation

This happens when materials change between liquid and gas states. Examples include water boiling into steam, or steam condensing into water.

For water, the specific latent heat of vaporisation is 2,260,000 J/kg.

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Notice that vaporisation needs much more energy than fusion! This is because gas particles are much further apart than liquid particles, requiring more energy to completely overcome intermolecular forces.

The equation

You can calculate the thermal energy needed using this equation:

$E = m \times L$

Where:

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

How is this different from specific heat capacity?

Understanding the difference between these two concepts is crucial for students learning about thermal energy.

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Critical Difference:

Specific heat capacity is about changing temperature
Specific latent heat is about changing state
During state changes, temperature never changes - it stays constant
All the energy goes into breaking or making bonds between particles

Worked examples

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Worked Example 1: Melting ice

Calculate the energy needed to melt 25 kg of ice at 0°C.

Step 1: Identify the type of change Ice → water (solid → liquid), so use fusion

Step 2: Apply the equation $E = m \times L = 25 \text{ kg} \times 334,000 \text{ J/kg}$

Step 3: Calculate the result $E = 8,350,000 \text{ J}$

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Worked Example 2: Boiling water

Calculate the energy needed to turn 40 kg of water into steam at 100°C.

Step 1: Identify the type of change Water → steam (liquid → gas), so use vaporisation

Step 2: Apply the equation
$E = m \times L = 40 \text{ kg} \times 2,260,000 \text{ J/kg}$

Step 3: Calculate the result $E = 90,400,000 \text{ J}$

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Key Points to Remember:

State changes happen at constant temperature
Fusion = solid ↔ liquid (melting/freezing)
Vaporisation = liquid ↔ gas (boiling/condensation)
Water fusion = 334,000 J/kg
Water vaporisation = 2,260,000 J/kg
Always use the correct L value for the type of change happening
Use the equation $E = m \times L$ to calculate energy needed
Vaporisation needs much more energy than fusion for the same material

Specific latent heat (AQA GCSE Physics): Revision Notes