Cambridge IGCSE Physics · Syllabus 0625 · 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, zero degrees Celsius for water at standard atmospheric pressure; the energy supplied separates the particles and frees them from their fixed positions rather than making them move faster, so the temperature stays constant during the change.
This definition is part of the Thermal Physics chapter in Cambridge IGCSE Physics.
Common mistakes with Melting
- 3. “Heat and temperature mean the same thing.” Core 2.2.2.1 Why it is wrong: one is an energy transfer, the other is a measure of particle motion. 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.”
- 5. “The temperature rises continuously while a pure substance melts or boils.” Core 2.2.3.1 Why it is wrong: it assumes supplied energy must always increase particle speed. During a change of state it separates the particles 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 they do not move any faster and the temperature stays constant.”
- 6. “Energy is needed to make a gas condense.” Core 2.2.3.3 Why it is wrong: it reverses the direction of the energy flow. Condensation and solidification bring particles closer together, which releases energy. Corrected model: moving towards the gas end — melting, boiling, evaporation — takes energy in. Moving the other way — condensation, solidification — gives energy out. Exam-safe: “As the gas condenses, the particles slow down and come close enough for the forces between them to hold them together, and energy is given out to the surroundings.”
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.