Interpret melting and boiling on a heating curve
Explain energy input during constant-temperature changes of state without claiming energy disappears.
Core and Extended; Physics 0625, 2026–2028, section 2.2.3. Explain energy input during melting/boiling, water's standard-pressure temperatures, condensation/solidification, surface evaporation and liquid cooling. Detailed boiling/evaporation comparisons, rate factors and cooling a contacted object belong to the labelled Supplement lessons. Written practice is self-checked
Before you begin
- Identify solid, liquid and gas states.
- Relate a temperature rise to internal energy.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
A pure-water curve is horizontal at 0°C while ice melts and input continues. What is happening?
What you will learn
- Describe melting and boiling as energy-input processes without a temperature change.
- Distinguish heating within a state from a phase-change plateau.
Read each stage of a specified model
The original heating curve shows a pure water sample at standard atmospheric pressure receiving energy in an idealised account. A sloping part shows temperature rising within a state. A horizontal part at 0°C shows melting; a later horizontal part at 100°C shows boiling. Both horizontal stages still receive energy while the material changes state.
During melting, solid and liquid can coexist; during boiling, liquid and gas can coexist. A single temperature does not tell the proportion in each state. Horizontal progress is schematic rather than a calibrated time or energy axis; plateau widths cannot compare actual durations or energy amounts. Real readings can depart from a perfect plateau because of mixture, pressure, temperature gradients, sensor response or energy-transfer conditions.
Explain where the energy goes
During the specified constant-temperature melting or boiling stage, incoming energy increases internal energy associated with changing particle interactions and arrangement. It does not produce the same temperature rise as warming within a single state. The average kinetic-energy explanation for a temperature rise should therefore not be applied as a continuing increase throughout this ideal plateau.
The thermometer's lack of rise does not mean the heater has stopped supplying energy or that conservation has failed. Nor does a plateau alone establish an energy amount in joules: a supplied input rate and time, with the relevant energy destinations stated, would be needed. Specific heat capacity describes a same-state temperature change and does not calculate the energy for this plateau.
Pause and explain
What can a 100°C boiling plateau alone tell you about the joules received?
Worked example
In a pure-water model at standard atmospheric pressure, energy enters steadily while temperature stays at 0°C and ice becomes liquid. Explain the plateau.
Show the worked solution
- The state changes from solid to liquid.
- Energy continues entering without a temperature rise.
- Internal energy changes through particle interactions/arrangement rather than a rising average kinetic-energy account.
Answer Melting uses energy at constant temperature in this model.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the 0°C and 100°C plateaus on the ideal pure-water heating curve at standard atmospheric pressure.
Give me a hint
Name each state change and keep continuing input explicit.
Compare my reasoning
- At 0°C the sample melts, with solid and liquid present during the change.
- At 100°C it boils, with liquid changing to gas.
- Both stages receive energy without a temperature rise in this model.
0°C: melting; 100°C: boiling; energy enters during both.
Look for these in your work
- Identify each transition.
- Separate temperature change from energy transfer.
The curve slopes upward while water remains liquid, then is horizontal during boiling. Compare the energy explanations.
Give me a hint
Contrast within-state warming and state change.
Compare my reasoning
- The sloping liquid stage has rising temperature and average particle kinetic energies.
- The boiling stage changes state at constant temperature under the specified conditions.
- Incoming energy still increases internal energy through changing interactions/arrangement.
Warming raises temperature; boiling changes state at constant temperature while receiving energy.
Look for these in your work
- Use the stated phases.
- Retain energy input on the plateau.
A heater supplies 40 W for 60 s entirely to a sample during a constant-temperature phase change in a supplied ideal account. Find input and evaluate a zero-energy claim.
Give me a hint
Use power × time; a plateau does not override that energy account.
Compare my reasoning
- Input energy is 40 × 60 = 2400 J.
- The problem assigns that energy entirely to the phase change.
- The constant temperature does not make the 2400 J zero or justify mcΔθ for the phase-change energy.
2400 J input; the zero-energy claim is false.
Look for these in your work
- Use the supplied ideal boundary.
- Do not use the sensible-heating equation for a plateau.
Common mistakes
- A horizontal temperature line does not mean zero input.
- A temperature does not specify the fraction melted.
- The illustrated times are not measured universal durations.
What can you explain now?
Does constant-temperature boiling require zero energy input?
Compare with the explanation
No.
Energy continues changing liquid into gas without raising the temperature in the stated model.
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Make it stick
Explain the sloping and horizontal stages without confusing constant temperature with zero energy transfer.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.