A heating curve plots temperature against time as a substance is heated steadily. The key feature is the flat section (plateau) at each change of state. Understanding why this happens is one of the most important mechanism questions in GCSE chemistry.
Temperature measures average kinetic energy. When particles speed up, temperature rises. During a change of state:
- Energy is being absorbed continuously (the substance is still being heated).
- But all of that energy goes into breaking the forces of attraction between particles — not into speeding them up.
- Because kinetic energy stays constant, temperature stays constant.
- Once all the substance has changed state, energy input starts increasing kinetic energy again and the temperature rises.
Key principle: During a change of state, energy is absorbed or released but temperature does not change — because the energy is changing potential energy (particle separation), not kinetic energy.
A full heating curve for a pure substance has three rising sections (solid warming, liquid warming, gas warming) and two flat plateaus (at the melting point and boiling point). Substances with stronger forces of attraction have longer plateaus because more energy is needed to break those forces.
This how it works covers Why Heating Curves Have Flat Sections within States of Matter for GCSE Chemistry. Revise States of Matter in Atomic Structure for GCSE Chemistry with 12 exam-style questions and 10 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 5 of 13 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for States of Matter
Which of the following best describes the arrangement of particles in a solid?
A heating curve shows a flat section (plateau) when a solid is being heated to become a liquid. Explain why the temperature does not rise during this flat section. [3 marks]