Interpret melting and boiling on a heating curve
Explain energy input during constant-temperature changes of state without claiming energy disappears.
Section 2.2.3 · Melting, boiling and evaporation
Exam scope and assessment notes
Core and Extended · Physics 0972, 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; human teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
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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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.