B.1.8—Phase change
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- HL
Explain Phase Change at Constant Temperature
What changes in a phase change
Melting, freezing, boiling, condensing and other phase changes alter how particles are arranged and how freely they move. Energy transfer changes the balance of intermolecular potential energy.
Why temperature stays constant
During a phase change of a pure substance at constant pressure, the supplied or removed energy changes particle interactions rather than increasing the average random kinetic energy. Therefore the temperature remains constant until the phase change is complete.
Read a heating curve
A sloped section represents temperature changing within one phase. A flat section represents energy transfer during a phase change. The flat section can be long even though the thermometer reading does not change.
Common trap
“Constant temperature” does not mean “no energy transfer”. It means the transfer is not increasing average particle kinetic energy at that stage.
B.1.8 Exam Analysis
Assessment in practice
1 marksHow it is assessed
The evidence uses heating-curve multiple choice to identify why temperature is constant and a phase-change table to compare internal energy with intermolecular potential energy.
Command terms
Explain / State / Determine
What earns marks
On a flat heating/cooling-curve section, state that temperature and therefore average molecular kinetic energy remain constant, while energy transfer changes intermolecular potential energy and particle arrangement. During freezing, internal energy and intermolecular potential energy decrease; during melting they increase.
Watch for
Claiming that constant temperature means no energy transfer or constant internal energy.
Representative question
Question 1
[Maximum number: 1]
A substance changes from a liquid into a solid without a change in temperature.
What is true about the internal energy of the substance and the total intermolecular potential energy of the substance when this phase change occurs?
Internal energy of
the substance
Total intermolecular potential
energy of the substance
decrease
decrease
no change
decrease
decrease
no change
no change
no change
A