Investigating Specific Latent Heat

Heating a substance normally increases its temperature — but during melting and boiling, that pattern suddenly stops. Students often find this confusing: why does temperature stay constant even though energy is still being added?

The answer lies in specific latent heat: the amount of energy needed to change the state of 1 kg of a substance without changing its temperature. This experiment helps GCSE and A Level Physics students measure latent heat directly and understand the energy involved in state changes.

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What Is Specific Latent Heat?

There are two types:

1. Latent Heat of Fusion (solid → liquid)

Energy needed to melt a substance at its melting point.

2. Latent Heat of Vaporisation (liquid → gas)

Energy needed to boil a substance at its boiling point.

The formula is:

$$Q = mL$$

where

• $Q$ is energy supplied (J)

• $m$ is mass changed (kg)

• $L$ is specific latent heat (J/kg)

This shows why boiling a kettle takes so much energy — most of it goes into breaking intermolecular bonds, not into raising temperature.

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Investigating Latent Heat of Fusion (Melting Ice)

Equipment:

• Crushed ice

• Beaker

• Immersion heater

• Ammeter and voltmeter

• Stopwatch

• Balance

Method:

1. Dry the ice to remove meltwater.

2. Place ice in a beaker and measure its mass.

3. Turn on the immersion heater and record current and voltage.

4. Time the heating for a fixed period (e.g. 5 minutes).

5. Measure the remaining mass of ice or mass of melted water.

6. Calculate electrical energy supplied:

   $$Q = IVt$$
   

7. Use $Q = mL$ to find $L$.

8. Have an identical setup with approximately the same amount of ice. Start and record this experiment simultaneously, but don't switch the heater on.

9. Compare the meltwater in each and subtract the control from the experimental value to determine the amount of ice melted by the heater.

This provides students with a practical understanding of the specific latent heat of fusion of ice (~334,000 J/kg).

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Investigating Latent Heat of Vaporisation (Boiling Water)

Equipment:

• Kettle or boiling water heater

• Ammeter and voltmeter (for an immersion heater setup) or a Joulemeter

• Balance

• Stopwatch

Method:

1. Heat water and allow it to boil steadily.

2. Before starting timing, measure the mass of the kettle.

3. Boil for a fixed period (e.g. 2–3 minutes).

4. Measure how much water was lost as steam (change in mass).

5. Calculate energy input using electrical power:

   $$Q = IVt$$
   

6. Have an identical setup with boiling water, but don't turn on the heater. Measure the loss in mass.

7. Take the control value away from the experimental value.

8. Use mass lost and energy supplied to calculate latent heat.

Typical result for water vaporisation:

$$L_v \approx 2.26 \times 10^6 \text{ J/kg}$$

Students immediately see why boiling takes so much energy compared to melting.

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Typical Student Results

Process	Mass Changed (kg)	Energy Supplied (J)	Calculated $L$ (J/kg)	Accepted Value
Melting ice	0.015	5100	340,000	334,000
Boiling water	0.010	23,000	2,300,000	2,260,000

These results are impressively close to accepted values if heating is well controlled.

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Why This Works in Teaching

Students see energy being supplied without a temperature change, which challenges the idea that “heat always makes things hotter.”

They learn that:

• melting and boiling require breaking bonds

• temperature plateaus represent energy being used internally

• large amounts of energy are involved in state changes

• electrical power and energy calculations underpin real measurements

It strengthens both conceptual understanding and required practical skills.

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Skills Highlight

• Measuring mass accurately

• Using $Q = IVt$ to calculate energy

• Handling experimental uncertainty (heat loss, evaporation)

• Calculating and interpreting specific latent heat

• Understanding energy transfer during state changes