Explain why the temperature of a substance does not change whilst it is boiling, even though it is continuously being heated.

Edexcel GCSE Physics (1PH0) — 14.1 Particle model · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A student heats a beaker of water until it reaches its boiling point. She notices that the thermometer reading stays constant throughout the boiling process, even though the Bunsen burner remains lit.

Model answer (4 marks)

During boiling the supplied heat is used to break the intermolecular forces that hold the liquid together, not to increase the kinetic energy of the particles. This energy is the latent heat of vaporisation. Because the kinetic energy – and therefore the temperature – does not rise, the thermometer reading remains constant. The energy instead raises the potential energy of the particles, allowing them to separate and form a gas.

Examiner tips

Use the term ‘latent heat of vaporisation’ and explain it is energy for changing state.
Show that temperature is a measure of kinetic energy, not potential energy.
Mention that the Bunsen burner supplies heat but it is absorbed in the phase change.
Keep the answer concise – 4 points are enough for full marks.

Common mistakes

Confusing latent heat with specific heat capacity.
Saying the temperature rises during boiling.
Not linking the constant temperature to kinetic energy staying constant.

Mark scheme (4 marks)

During boiling, the energy supplied goes into changing state (breaking intermolecular forces / overcoming forces between particles) rather than increasing the kinetic energy of the particles.
This energy is the latent heat (specific latent heat of vaporisation) of the substance.
Because the kinetic energy of the particles does not increase, the temperature does not rise.
The energy instead increases the potential energy of the particles, allowing them to separate and become a gas.

Key terms in this question

temperature