Heating curves and gas-particle explanations
Explain flat regions of a heating curve and gas volume changes using energy and particle collisions.
Extended only (Supplement) · Chemistry 0620, 2026–2028.
Before you begin
- Know the arrangement and movement of particles in each state.
- Recognise that melting and boiling change state without changing chemical identity.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
During boiling, what happens to chemical identity?
What you will learn
- Interpret heating and cooling curves using particle energy.
- Explain how temperature and pressure affect gas volume under stated conditions.
A plateau is still an energy transfer
For a pure substance at constant pressure, temperature can remain constant while melting or boiling takes place. Energy supplied during the plateau changes the arrangement and separation of particles by overcoming attractions, rather than increasing their average kinetic energy. During a rising segment within one state, particles gain average kinetic energy and temperature rises.
On cooling, energy leaves the sample. During freezing or condensation, attractions form as particles move into a more closely associated arrangement; temperature can stay constant while the state change proceeds. Do not assume every real mixture gives perfectly flat plateaus.
Collisions connect temperature, pressure and volume
Gas pressure comes from particles colliding with the walls. Heating makes the particles move faster on average. With a fixed volume, wall collisions become more frequent and more forceful, so pressure rises. With a movable piston under constant outside pressure, the gas expands until its pressure balances that outside pressure again.
At fixed temperature, increasing the applied pressure pushes a movable boundary inward. The smaller volume brings particles closer together and increases collisions with the walls. The number of particles and their size do not change. The syllabus requires this explanation; do not replace it with an unsupported formula.
Pause and explain
A pure liquid boils at constant pressure while its heater stays on. Why does temperature stay constant?
Worked example
A pure substance is heated. Its temperature rises to 60°C, stays at 60°C for four minutes, then rises again. What happens during the flat segment if it is melting?
Show the worked solution
- The sample still receives thermal energy.
- Solid and liquid coexist while the state change takes place.
- The supplied energy overcomes attractions rather than raising average kinetic energy.
Answer Melting continues at constant temperature while energy changes particle arrangement.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A heating curve has a rising solid segment, a flat melting segment and a rising liquid segment. Explain the energy change in each.
Give me a hint
Separate temperature changes from state changes.
Compare my reasoning
- The solid segment raises average kinetic energy.
- The melting plateau uses energy to overcome attractions and change arrangement.
- The liquid segment again raises average kinetic energy.
Rising segments: average kinetic energy increases. Melting plateau: energy changes arrangement.
Look for these in your work
- All three segments are addressed.
- Energy input is recognised throughout heating.
A pure liquid cools and then freezes at a constant temperature. Explain the flat part without saying it stops losing energy.
Give me a hint
Consider the formation of attractions.
Compare my reasoning
- Energy continues to leave the sample.
- Particles form a fixed solid arrangement as attractions become established.
- Temperature stays constant while liquid and solid coexist during freezing.
Energy leaves during freezing even though the temperature stays constant.
Look for these in your work
- The direction of energy transfer is correct.
- The explanation describes a state change rather than no energy transfer.
Two identical gas samples are heated: one in a rigid container, one under a freely moving piston at constant pressure. Explain their different responses.
Give me a hint
Use wall collisions and the constraint on each container.
Compare my reasoning
- In both samples, particles move faster on average.
- The rigid sample cannot expand, so stronger and more frequent wall collisions raise pressure.
- The piston moves outward, increasing volume until pressure again balances the outside pressure.
Rigid: pressure rises. Movable piston: volume rises while pressure stays constant.
Look for these in your work
- Particle motion explains both cases.
- Different boundary conditions lead to different predictions.
Common mistakes
- Saying the heater stops supplying energy during a plateau.
- Explaining increased gas pressure by enlarged particles.
What can you explain now?
Why does pressure rise when a sealed rigid gas container is heated?
Compare with the explanation
Faster particles collide more frequently and more forcefully with the walls.
The container fixes the volume, so the increased average kinetic energy raises pressure rather than expanding the container.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Draw a heating curve from memory, label the state in every segment and explain where the energy goes. Then explain one gas change with its fixed condition.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.