IB Physics SL B.1.8 Phase change
Explain constant-temperature melting, boiling and freezing by linking phase-change plateaus on graphs to energy stored in bonds.
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- SL
Exam points
- read a T-time or T-energy graph plateau and state that average molecular kinetic energy stays constant
- explain that energy supplied or removed changes intermolecular potential energy during the phase change
- choose or sketch a T-Q graph with a flat section at the melting or freezing point
B.1.8—Phase change question 1
[Maximum number: 3]
This section consists of three questions: B1, B2 and B3.
The water in (b) is further heated until it starts to boil at constant temperature. It is boiled for 500 s measured from the time that it first starts to boil. The mass of water remaining after this time is 0.20 kg .
Explain, in terms of the energy of the molecules of the water, why the water boils at constant temperature.
All the thermal energy is used to separate the molecules / break the bonds between molecules.
It does not increase their average kinetic energy.
Average kinetic energy is a measure of the temperature of the water.
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