Explain condensation and solidification using particles
Trace gas-to-liquid and liquid-to-solid changes without destroying particles or stopping all motion.
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Describe solid, liquid and gas particle models.
- Identify the direction of a change of state.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which change is condensation?
What you will learn
- Describe condensation and solidification using particle arrangement and motion.
- Distinguish energy transfer out from a compulsory temperature drop during the transition.
Condensation brings particles into a liquid arrangement
Condensation is gas becoming liquid. The gas particles are widely separated and move between collisions; in the liquid they are close together and can still move between positions. The particles do not disappear or become a different chemical substance. In the chosen same-material closed comparison, particle identity and count are retained while the state and arrangement change.
Energy transfers out during the condensation change in the stated cooling account. The visible drops on a cool surface are liquid formed from vapour; the vapour itself is not the white mist. A before/after particle diagram is schematic and does not supply real particle sizes, densities or an exact temperature history.
Solidification retains vibration
Solidification, including freezing for water, is liquid becoming solid. Particles become arranged about relative positions and vibrate there rather than moving freely between positions as in a liquid. Solid particles are not motionless; ordinary freezing is not absolute zero. Keep the distinction between microscopic vibration and the bulk object staying still.
In a pure-material fixed-pressure model, energy can leave during solidification while temperature stays constant at the transition. Cooling before the transition can lower temperature, but the temperature need not keep falling throughout the plateau. Describe energy transfer out and the particle change rather than asserting that every step must decrease the thermometer reading.
Sources for this lesson
Pause and explain
Which account correctly describes solidification?
Worked example
Explain liquid water becoming ice without saying its molecules vanish or stop all motion.
Show the worked solution
- The change is liquid to solid.
- Particles take relative positions and continue vibrating.
- In a specified transition plateau, energy leaves without a continuing temperature fall.
Answer Solidification changes arrangement and permitted motion; particles remain and vibrate.
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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.
Describe condensation in a same-material closed particle model.
Give me a hint
Retain particles while changing their arrangement.
Compare my reasoning
- Begin with widely separated gas particles.
- They become close together in a liquid arrangement.
- Particles remain able to move between positions; they are not destroyed.
Gas becomes a close, mobile liquid arrangement with particle identity retained.
Look for these in your work
- State the direction.
- Retain the liquid's permitted motion.
Compare the motion of particles before and after a liquid solidifies.
Give me a hint
Contrast positional mobility with vibration.
Compare my reasoning
- Liquid particles can move between positions.
- In the solid they vibrate about relative positions.
- The change does not mean zero microscopic motion.
Mobile liquid particles become constrained vibrating solid particles.
Look for these in your work
- Use both states' motion.
- Reject the motionless-solid claim.
A pure sample solidifies at constant temperature in a fixed-pressure model. Someone argues no energy can leave unless temperature falls. Explain.
Give me a hint
A state-change plateau separates transfer from temperature fall.
Compare my reasoning
- Energy can leave during solidification.
- Particle interactions and arrangement change.
- The specified plateau does not require a falling thermometer reading throughout the transition.
Energy leaves during the state change even while temperature stays constant.
Look for these in your work
- Respect the stated transition model.
- Do not infer zero transfer from a plateau.
Common mistakes
- Condensation is gas to liquid, not liquid to gas.
- Solidification does not destroy molecules.
- Freezing does not make particles motionless.
What can you explain now?
What is wrong with 'solid particles stop moving when freezing finishes'?
Compare with the explanation
Solid particles still vibrate about relative positions.
Being a solid describes constrained motion and arrangement, not total absence of microscopic motion.
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Make it stick
Describe condensation and solidification with direction, arrangement, retained particles and appropriate energy transfer.
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.