Phase transitions
During a phase transition such as melting or boiling, the energy added goes into breaking bonds between particles rather than raising the temperature, which stays constant until the transition is complete.
Practise this conceptHeating ice steadily produces a graph with a surprising flat spot: the temperature climbs toward 0°C, then stops rising completely while the ice melts, then resumes climbing once it's all liquid water. The energy didn't stop flowing in — it just stopped raising the temperature.
That energy is going into breaking the bonds holding water molecules in their rigid ice structure, not into speeding the molecules up. Since temperature reflects the particles' average kinetic energy, and the energy during melting is being spent on bonds rather than speed, the temperature holds steady until every last bit of ice is gone.
The energy required per kilogram to complete a phase change is called the latent heat, and it can be surprisingly large — turning boiling water into steam takes roughly seven times more energy than heating that same water all the way from freezing to boiling, which is exactly why steam burns are so much worse than hot-water burns.
Key ideas
Requires: Specific heat capacity
Formulas
| Symbol | Name | Unit |
|---|---|---|
| heat | J (joule) | |
| mass | kg (kilogram) | |
| heat of fusion | J/kg (joule per kilogram) |
Common misconceptions
- The temperature of a substance keeps rising while it melts or boils.
- Melting and freezing happen at different temperatures for the same pure substance.
- The energy added during a phase transition increases the average kinetic energy of the particles.