Wait, What? You can keep adding energy to a substance while its temperature barely changes.
During a phase change, energy can go into changing the arrangement and separation of particles rather than increasing average kinetic energy. That is why a temperature plateau can coexist with continuing energy transfer—and why latent-heat practicals require a different measurement strategy from specific heat capacity.
The scientific job
For a phase change at appropriate conditions,
ΔE = mL
where L is specific latent heat and m is mass changed from one phase to another. A practical therefore needs an estimate of energy responsible for phase change and a measurement of corresponding changed mass.
The Institute of Physics includes measuring the specific latent heat of fusion of ice as an advanced practical and emphasises that energy is transferred during phase change without the usual temperature rise. IOPSpark: Latent heat.
Why E = Pt is not automatically the phase-change energy
If an electrical heater supplies power P for time t, electrical energy is approximately E = Pt or VIt. But not all that energy necessarily melts ice or vaporises liquid. Some warms apparatus; some enters from or leaves to the surroundings; some may warm meltwater away from the transition temperature.
The core practical skill is therefore energy accounting.
A stronger ice method uses a control
Consider ice near 0 °C draining meltwater into a beaker. In the experimental setup an electrical heater supplies energy; in a matched control, ice melts from environmental heat alone. Over the same time interval, the difference in collected meltwater mass estimates the mass melted by the heater.
If m₁ is experimental meltwater and m₂ is control meltwater, then approximately:
mheater = m₁ − m₂
and L ≈ E/(m₁ − m₂).
This subtraction is powerful because it converts “heat from the surroundings” from a vague limitation into a measured correction.
Why the ice should already be near melting temperature
If ice begins far below 0 °C, some heater energy first raises its temperature. That is sensible heating, not latent heat of fusion. The simple E = mL model then over-assigns energy to melting and can make L appear too large.
Mass measurement can be the limiting signal
If only a few grams melt, balance resolution and retained water droplets become significant. Meltwater can cling to apparatus; ice can carry surface water into the collection; splashing and evaporation can change measured mass. Longer safe measurement intervals increase mass signal but also increase environmental exchange.
Quantitative window
A heater transfers 6000 J during a timed interval. Experimental meltwater is 25.2 g; control meltwater is 7.5 g.
Heater-attributed melt mass = 17.7 g = 0.0177 kg.
L ≈ 6000 / 0.0177 ≈ 3.39 × 10⁵ J kg⁻¹.
The value is plausible for ice, but plausibility is not proof. The control must actually represent environmental melting in the experimental apparatus.
Alternative: vaporisation and mass loss
For boiling, electrical energy can be related to mass lost as vapour. But evaporation also occurs below boiling, droplets may escape mechanically, and heat loss to surroundings can be substantial. A control or comparative power method can improve the energy ledger.
Temperature plateau: useful evidence, not perfect proof
A flat region in a heating curve supports the idea that energy is being absorbed during phase change, but real samples may show sloped or broadened transitions because of impurities, sensor lag, pressure variation and simultaneous heating of apparatus.
Observation versus inference
Observation: the heater supplied 6000 J and the corrected additional meltwater mass was 17.7 g.
Transformation: E/m gave 3.39 × 10⁵ J kg⁻¹.
Inference: this estimates specific latent heat of fusion under the experimental model.
Boundary: it assumes the correction adequately accounts for non-phase-change energy pathways.
Failure modes
- Ice starts below the melting point.
- Surface water is counted as newly melted ice.
- No environmental-melting control.
- Control and experimental setups have different exposure or geometry.
- Electrical power drifts but is treated as constant.
- Mass loss by splashing or evaporation is misidentified as the target phase change.
Checkpoint 1
A student ignores 8 g of meltwater produced in an unheated control and uses all 28 g from the heated apparatus. What happens to calculated L?
Checkpoint 2
Ice begins at −12 °C. Why does directly using E/m for the melted mass tend to overestimate L?
Answer key and WHY reasoning
1: m is too large for the heater-attributed melting, so E/m is too small. The calculated L is underestimated.
2: part of E warmed the ice from −12 °C toward 0 °C. Assigning that sensible-heating energy to phase change makes the numerator too large for the actual latent process, biasing L upward.
Unfamiliar transfer: freezing instead of melting
Latent heat is released during freezing. A cooling curve can reveal a phase-change region, but extracting L requires accounting for energy leaving the system, container heat capacity and changing heat-transfer rate. The direction of energy transfer reverses; the accounting logic remains.
Secondary → JC → deeper Science
Secondary: phase changes, temperature plateaus, E = mL and qualitative energy transfer.
JC: electrical energy measurement, controls, corrected mass change, uncertainty and separation of latent from sensible heating.
Deeper Science: calorimetry expands to differential scanning calorimetry, enthalpy, phase diagrams, nucleation, impurities and pressure-dependent transitions.
How to study this practical
Draw an energy ledger with arrows: heater → phase change, apparatus, surroundings and sensible heating. Then label which arrows are measured, corrected, assumed negligible or unknown. If you cannot account for an arrow, you have found an evaluation point.
Evidence boundaries
A school practical estimates L under stated conditions. It does not show that phase change is perfectly isothermal in every real material or that L is independent of pressure, purity and experimental state.
Authoritative next steps
Teaching Guide
For teachers and parents: make the student identify what the heater energy can do besides melt the sample. Then introduce the control and ask what quantity its subtraction is trying to estimate. This turns a formula practical into experimental energy accounting.