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Why does the temperature stay constant while ice melts or water boils, even though heat is still being supplied?

Describe melting, boiling, and evaporation, and explain latent heat using the particle model

A focused answer to the O-Level Physics outcome on changes of state and latent heat. Why temperature stays constant during a change of state, the difference between boiling and evaporation, and latent heat in terms of particle bonds.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

SEAB wants you to describe melting, boiling, and evaporation, to explain why temperature stays constant during a change of state, and to explain latent heat in terms of the energy needed to break the bonds between particles. The big idea is that during a change of state the supplied energy separates particles rather than speeding them up, so the temperature does not change.

The answer

Melting and boiling at constant temperature

When a pure solid melts, or a pure liquid boils, the temperature stays constant even though heat is still being supplied. Ice melts at 0 ∘C0\,^\circ\text{C} and water boils at 100 ∘C100\,^\circ\text{C} (at standard atmospheric pressure), and these temperatures hold steady throughout the change.

Latent heat and the particle model

The energy supplied during a change of state does not raise the temperature. Instead it does work to break the bonds holding the particles together, separating them into the new state. This hidden energy is called latent (hidden) heat.

  • Melting: energy breaks the bonds fixing particles in the solid, freeing them to slide as a liquid.
  • Boiling: energy separates the liquid particles completely so they fly apart as a gas.

Because the energy goes into separating particles (their potential energy) rather than speeding them up (their kinetic energy), the temperature stays constant.

Boiling versus evaporation

Both turn liquid into gas, but they differ:

Boiling Evaporation
Temperature Only at the boiling point At any temperature
Where Throughout the liquid Only at the surface
Bubbles Yes No
Speed Fast Slow

Why evaporation causes cooling

In a liquid, particles have a range of speeds. During evaporation, the fastest (most energetic) particles escape from the surface. This leaves behind the slower, lower-energy particles, so the average energy, and hence the temperature, of the remaining liquid falls. That is why evaporation cools.

Examples in context

Example 1. Steam burns. Steam at 100 ∘C100\,^\circ\text{C} scalds far worse than water at 100 ∘C100\,^\circ\text{C}, because when the steam condenses on your skin it releases its large latent heat of vaporisation in addition to cooling from 100 ∘C100\,^\circ\text{C}. The hidden energy stored when the water boiled is given back all at once.

Example 2. Cooling by sweating. When you exercise, you sweat, and the sweat evaporates from your skin. The most energetic water particles leave first, carrying energy away and lowering the temperature of the skin. This is the body's natural use of evaporative cooling to prevent overheating.

Try this

Q1. State what happens to the temperature of pure ice while it is melting. [1 mark]

  • Cue. It stays constant at 0 ∘C0\,^\circ\text{C} throughout the melting.

Q2. State two ways boiling differs from evaporation. [2 marks]

  • Cue. Boiling is at a fixed temperature throughout the liquid (with bubbles); evaporation is at any temperature from the surface only (no bubbles).

Q3. Explain, in terms of particles, why the temperature stays constant during boiling. [2 marks]

  • Cue. The supplied energy separates the particles into a gas (breaking bonds), not speeding them up, so the kinetic energy and hence the temperature do not change.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksIce at 0 ∘C0\,^\circ\text{C} is heated steadily until it has all melted into water at 0 ∘C0\,^\circ\text{C}. (a) State what happens to the temperature during melting. (b) Explain, using the particle model, where the supplied energy goes.
Show worked answer →

(a) The temperature stays constant at 0 ∘C0\,^\circ\text{C} throughout the melting, even though heat is still being supplied.

(b) The supplied energy is used to break the bonds holding the particles in their fixed positions in the solid, not to increase their kinetic energy. So the energy goes into separating the particles (potential energy) rather than raising the temperature.

Markers reward the constant temperature during melting, and the explanation that the energy breaks the bonds between particles rather than increasing their speed, so the temperature does not rise.

Original4 marks(a) State two differences between boiling and evaporation. (b) Explain why sweating helps to cool the body.
Show worked answer →

(a) Boiling happens at a fixed temperature (the boiling point) throughout the whole liquid, with bubbles forming; evaporation happens at any temperature only at the surface of the liquid, with no bubbles. (Any two distinct differences.)

(b) When sweat evaporates from the skin, the fastest (most energetic) particles escape, leaving behind the slower, lower-energy particles, so the remaining liquid and the skin cool down. Evaporation takes energy from the skin, cooling the body.

Markers reward two valid differences (fixed point versus any temperature; throughout versus surface only; bubbles versus none), and the cooling explanation that the most energetic particles leave during evaporation, lowering the average energy of those left.

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