5 · IGCSE Physics
Heating curve and changes of state
Heat ice at −20 °C with a steady heater until it becomes steam at 120 °C. Watch the temperature–time graph, the particles, and which energy equation applies at each stage.
E = P t (all heater energy goes into the H₂O)
Temperature changing: E = m c Δθ
Changing state: E = m L
c: ice 2100, water 4200, steam 2010 J kg⁻¹ °C⁻¹. L (fusion) = 3.34 × 10⁵ J kg⁻¹, L (vaporisation) = 2.26 × 10⁶ J kg⁻¹
| Stage | Uses | Energy |
|---|---|---|
| Ice −20 → 0 °C | mcΔθ | |
| Melting at 0 °C | mL | |
| Water 0 → 100 °C | mcΔθ | |
| Boiling at 100 °C | mL | |
| Steam 100 → 120 °C | mcΔθ |
Try this
- Compare the two flat sections: the boiling section should be about 6.8 times longer than the melting section. Check this against the two values of L.
- Compare the slopes for ice and for water. Why does water warm at about half the rate of ice with the same heater?
- Double the mass, then double the power. Predict how the total time changes in each case before you press Play.
Common error: saying the temperature rises while ice melts or water boils; on a flat section the energy increases the potential energy of the particles (use E = mL), and only on sloping sections do you use E = mcΔθ.