Cambridge O Level Physics · Syllabus 5054 · Thermal Physics
Boiling
What is Boiling?
Boiling is the change of state from liquid to gas that occurs throughout the whole liquid at a definite temperature for a given pressure, one hundred degrees Celsius for water at standard atmospheric pressure, with bubbles of vapour forming inside the liquid and the temperature remaining constant while the change takes place.
This definition is part of the Thermal Physics chapter in Cambridge O Level Physics.
Common mistakes with Boiling
- 6. “The temperature rises continuously while a pure substance melts or boils.” Why it is wrong: it assumes supplied energy must always increase particle speed. During a change of state it increases particle potential energy instead. Corrected model: the temperature stays constant throughout melting and throughout boiling, giving the flat sections of a heating curve. Exam-safe: “The energy supplied separates the particles against the forces between them, so their average kinetic energy and therefore the temperature stay constant.”
- 7. “Boiling and evaporation are identical.” Why it is wrong: they differ in location, temperature condition, the presence of bubbles and their effect on the liquid's temperature. Corrected model: boiling occurs throughout the liquid at one definite temperature; evaporation occurs at the surface at any temperature and cools the liquid. Exam-safe: “Boiling occurs throughout the liquid at a definite temperature, whereas evaporation occurs only at the surface and at any temperature.”
Examiner tips on Boiling
- Command-word cue “State the change of state” wants one word: melting, solidification, boiling, condensation or evaporation. Writing “it turns into a liquid” describes the outcome but does not name the process, so it does not answer a question that asked for the name.
Questions students ask about Boiling
Why does the temperature stay constant while a pure substance melts or boils?
On a heating curve the temperature is constant during melting and during boiling because the energy supplied is separating the particles against the forces between them, not speeding them up. Since temperature depends on the average kinetic energy of the particles, and that average is not increasing during the change, the temperature does not rise until melting or boiling is complete. That energy is the latent heat.
What is the difference between boiling and evaporation?
Boiling happens throughout the whole liquid, only at one definite temperature — 100 °C for water at standard atmospheric pressure — with bubbles of vapour forming inside it. Evaporation happens only at the surface, can occur at any temperature below the boiling point, and cools the liquid left behind because the most energetic particles escape, lowering the average kinetic energy of those that remain. The two are not interchangeable.