Cambridge IGCSE Physics · Syllabus 0625 · 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 IGCSE Physics.
Boiling in context
Topic 2 of Cambridge IGCSE Physics 0625 contains 45 printed statements: 24 Core, taught to all candidates, and 21 Supplement, taught in addition to Core for candidates following the Extended route. The Core statements cover the kinetic particle model and states of matter, Brownian motion as evidence for that model, gas pressure and the qualitative gas relationships, the kelvin scale, thermal expansion and its applications, internal energy, changes of state and evaporation, and thermal energy transfer by conduction, convection and infrared radiation with basic everyday applications. The Supplement statements add inter-particle forces, force per unit area in gases, \(pV = \text{constant}\), the particle explanation of expansion, average kinetic energy and specific heat capacity with its two experiments, the boiling–evaporation comparison and evaporative cooling of an object, the mechanisms and limits of conduction, the radiation balance including the Earth's temperature, emitter and absorber experiments, and complex multi-route applications.
Heating a substance normally increases the average separation of its particles, so the substance expands; Extended candidates also compare how much each state expands. A rise in temperature increases an object's internal energy. When a pure substance melts or boils, the energy supplied separates the particles against the forces between them instead of speeding them up, so the temperature stays constant during the change; condensation and solidification are the reverse, with particles coming closer together and energy being given out. Evaporation is different again: it happens only at the surface, at any temperature below the boiling point, and it cools the liquid left behind.
Common mistakes with Boiling
- 5. “The temperature rises continuously while a pure substance melts or boils.” Core 2.2.3.1 Why it is wrong: it assumes supplied energy must always increase particle speed. During a change of state it separates the particles 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 they do not move any faster and the temperature stays constant.”
- 6. “Energy is needed to make a gas condense.” Core 2.2.3.3 Why it is wrong: it reverses the direction of the energy flow. Condensation and solidification bring particles closer together, which releases energy. Corrected model: moving towards the gas end — melting, boiling, evaporation — takes energy in. Moving the other way — condensation, solidification — gives energy out. Exam-safe: “As the gas condenses, the particles slow down and come close enough for the forces between them to hold them together, and energy is given out to the surroundings.”
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.