Cambridge O Level Physics · Syllabus 5054 · Thermal Physics
Melting
What is Melting?
Melting is the change of state from solid to liquid, which occurs at a fixed temperature for a pure substance at constant pressure; the energy supplied separates particles and weakens the forces holding them in fixed positions rather than raising their average kinetic energy, so the temperature stays constant during the change.
This definition is part of the Thermal Physics chapter in Cambridge O Level Physics.
Common mistakes with Melting
- 3. “Heat and temperature mean the same thing.” Why it is wrong: one is an energy transfer, the other is a measure of average particle kinetic energy. Adding energy to melting ice raises no temperature at all. Corrected model: thermal energy is transferred because of a temperature difference; temperature is what that transfer may or may not change. Exam-safe: “Thermal energy is transferred from the hotter object to the cooler one because of the temperature difference between them.”
- 6. “The temperature rises continuously while a pure substance melts or boils.” Why it is wrong: it assumes supplied energy must always increase particle speed. During a change of state it increases particle potential energy instead. Corrected model: the temperature stays constant throughout melting and throughout boiling, giving the flat sections of a heating curve. Exam-safe: “The energy supplied separates the particles against the forces between them, so their average kinetic energy and therefore the temperature stay constant.”
Examiner tips on Melting
- Command-word cue “State the change of state” wants one word: melting, solidification, boiling, condensation or evaporation. Writing “it turns into a liquid” describes the outcome but does not name the process, so it does not answer a question that asked for the name.
Questions students ask about Melting
Why does the temperature stay constant while a pure substance melts or boils?
On a heating curve the temperature is constant during melting and during boiling because the energy supplied is separating the particles against the forces between them, not speeding them up. Since temperature depends on the average kinetic energy of the particles, and that average is not increasing during the change, the temperature does not rise until melting or boiling is complete. That energy is the latent heat.