During a phase change, added energy increases the intermolecular potential energy of a substance rather than the average kinetic energy of its particles. Since temperature measures average particle kinetic energy, the temperature stays constant until the phase change is complete.
The Mechanism
This concept is part of B.1 Thermal energy transfers and applies to both SL and HL. When a pure substance is heated without changing phase, the supplied energy usually increases the particles’ average kinetic energy, so its temperature rises.
During melting or boiling, however, energy is transferred as latent heat. It overcomes attractive intermolecular forces, allowing particles to become more separated or rearranged. The substance’s intermolecular potential energy therefore increases without an increase in average kinetic energy.
The required energy is calculated using
where is transferred energy, is mass, and is the specific latent heat. Two relevant cases are:
| Phase change | Quantity used |
|---|---|
| Melting or freezing | Specific latent heat of fusion, |
| Boiling or condensation | Specific latent heat of vaporization, |
For example, melting of ice at , using , requires
The ice absorbs , but remains at while melting. Once all the ice becomes liquid, further energy increases the water’s average kinetic energy and temperature.
The process reverses during freezing or condensation: energy is released as particles enter a lower-potential-energy arrangement, while the temperature remains constant. This constant-temperature statement assumes a pure substance undergoing a phase change at constant pressure.
Exam Technique
For an IB Explain question, form a causal chain: energy is transferred, intermolecular potential energy changes, average kinetic energy remains constant, and therefore temperature remains constant. A common misconception is that no energy is absorbed along the flat section of a heating curve. Energy is still absorbed, but it changes particle separation or arrangement rather than temperature.