Overview
What you’ll learn
- Compare the physical properties of solids, liquids and gases.
- Use the kinetic particle model to describe the states of matter in terms of the arrangement, motion, forces and distances between particles.
- Relate a rise in temperature to the increase in average kinetic energy of the particles.
- Describe internal energy as the total kinetic and potential energies of the particles.
- Describe melting, solidification, boiling and condensation as processes of energy transfer without a change in temperature.
Key points
- 1Three states: solids keep shape and volume; liquids keep volume but take the container's shape; gases fill the whole container.
- 2The kinetic particle model describes each state by arrangement, separation, motion and forces between particles.
- 3Solid particles vibrate about fixed positions; liquid particles slide past one another; gas particles move fast and randomly, far apart.
- 4Temperature rise = increase in the ==average kinetic energy== of the particles.
- 5Internal energy = total kinetic + total potential energy of all the particles.
- 6Melting, solidification, boiling and condensation transfer energy at constant temperature — the energy changes particle potential energy, not average KE.
Notes · 1 of 5
Comparing solids, liquids and gases
Shape, volume, flow, density and compressibility side by side.
The three states behave differently in everyday ways, and the particle model (next section) explains every line of this comparison:
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed | Takes container’s shape | Fills whole container |
| Volume | Fixed | Fixed | No fixed volume |
| Flow? | No | Yes | Yes |
| Density | Usually highest | High | Very low |
| Compressibility | Almost incompressible | Almost incompressible | Easily compressed |
Two contrasts examiners love: density (a gas is hundreds of times less dense than the same substance as a liquid — its particles are spread far apart) and compressibility (syringe of air squashes easily; syringe of water barely budges).
Notes · 2 of 5
The kinetic particle model
Arrangement, separation, motion and forces — state by state.
Solid. Particles are packed tightly in a regular, fixed pattern, touching their neighbours. Strong forces hold each particle in place, so all a particle can do is vibrate about its own fixed position — it never swaps places with a neighbour. That is why solids keep both their shape and their volume.
Liquid. Particles remain close together (mostly touching) but the arrangement is irregular and constantly changing. The forces are weaker in effect: particles can slide and move past one another, so the liquid flows and takes its container’s shape while keeping a fixed volume — the particles stay just as crowded.
Gas. Particles are far apart, in a random ever-changing arrangement, with negligible forces between them. They travel fast and randomly in all directions, colliding with each other and the walls, until they have spread through the entire container — no fixed shape, no fixed volume.
Notes · 3 of 5
Temperature and kinetic energy
A thermometer reads average particle motion.
Particles never sit still. At any temperature they jostle and fly about with a spread of speeds. Temperature measures the average kinetic energy of the particles: heat the substance and the particles speed up, so the average KE rises — and the thermometer climbs. Cool it, and the average KE falls. Two beakers at the same temperature hold particles with the same average KE, even if one beaker holds far more water than the other.
Watch the word average: individual particles move slower and faster, but temperature reports the average. Higher temperature ⇒ higher average kinetic energy — that one line is the whole learning outcome.
Notes · 4 of 5
Internal energy
The grand total of particle kinetic and potential energy.
Internal energy is the total kinetic and potential energies of all the particles in a substance. The kinetic part comes from particle motion (tracked by temperature). The potential part comes from the forces between particles: pulling particles further apart — as happens during melting or boiling — stores more potential energy, just like stretching a spring.
Notes · 5 of 5
Changes of state at constant temperature
Melting, solidification, boiling, condensation — flat on the heating curve.
Four changes of state appear in this course, and they share one signature: the temperature stays constant while the change happens. Melting (solid → liquid) occurs at the melting point; solidification (liquid → solid) at the same temperature; boiling (liquid → gas) at the boiling point; condensation (gas → liquid) at that same boiling temperature. During each one, the energy being transferred is used to change the potential energy of the particles (pulling them apart or letting them lock together) — the average kinetic energy is unchanged, so the thermometer does not move.
The heating curve tells the whole story. Imagine ice heated steadily: the temperature climbs (particles vibrate faster), then flattens at 0 °C while the ice melts, then climbs again as liquid water warms, then flattens at 100 °C while the water boils, then climbs once more as steam warms. Flat section = change of state; sloping section = temperature rising. Energy is transferred in during melting and boiling, and out during solidification and condensation — always at constant temperature.
Revise
Key terms
The words examiners expect you to use precisely.
- kinetic particle model
- A model treating matter as particles in constant motion; the arrangement, separation, motion and forces between particles explain the properties of each state.One model explains shape, volume, flow, density and compressibility differences.
- arrangement
- How the particles are positioned: regular and fixed (solid), close and irregular (liquid), far apart and random (gas).First of the four beats in a full particle-model description.
- internal energy
- The total kinetic and potential energies of all the particles in a substance.Heating raises it; temperature is only the average kinetic part.
- average kinetic energy
- The mean energy of particle motion; what temperature measures.A temperature rise is a rise in average kinetic energy.
- melting point
- The constant temperature at which a solid melts (and a liquid solidifies).Water: 0 °C. Shown as the first flat section of a heating curve.
- boiling point
- The constant temperature at which a liquid boils (and a gas condenses).Water: 100 °C. The second flat section of a heating curve.
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- solidification
- The change from liquid to solid at constant temperature, transferring energy out.Molten wax hardening into a solid shape.
- condensation
- The change from gas to liquid at constant temperature, transferring energy out.Steam cooling back to water at 100 °C.
- heating curve
- A temperature–time graph for steady heating; flat sections mark changes of state.Slopes = warming; flats = melting or boiling at constant temperature.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Saying "the particles expand" when a substance is heated.
Particles do not grow; the object expands.
Instead: Heated particles move faster and push further apart on average — the object expands, the particles stay the same size.
Mistake: Using "heat" and "temperature" as the same thing.
Temperature measures the average kinetic energy of particles; internal energy is the total.
Instead: A bathtub at 30 °C has far more internal energy than a cup at 30 °C — same average, more particles.
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Mistake: Drawing a rising temperature line through a melting or boiling section.
Changes of state happen at constant temperature.
Instead: On a heating curve, melting and boiling are flat sections at the melting and boiling points.
Mistake: Claiming particles stop moving when a substance is cold.
Solid particles still vibrate — less vigorously, but they never rest.
Instead: Lower temperature = smaller average KE = gentler vibration.
Mistake: Describing gas particles as "floating upwards".
Gravity is not the story; gas motion has no preferred direction.
Instead: Gas particles move fast and randomly in all directions, filling the container top to bottom.
Mistake: Giving liquids the wide spacing of gases.
Liquid particles stay close together — nearly as crowded as solids.
Instead: Liquids are incompressible and dense because their particles touch; they merely slide past one another.
Mistake: Defining internal energy as only the kinetic energy of particles.
Half the definition is missing.
Instead: Internal energy = kinetic + potential energies of the particles; the potential part grows when particles are pulled apart during melting or boiling.
Mistake: Saying boiling happens only at the surface.
That confuses boiling with a surface-only process.
Instead: Boiling happens throughout the liquid at the boiling point — bubbles of vapour form everywhere and rise.
Mistake: Believing energy stops being transferred during a flat section.
The heater is still on — energy still flows.
Instead: During melting/boiling energy is transferred in but goes into particle potential energy, not average KE.
Mistake: Thinking condensation makes the temperature plunge.
Condensation happens at constant temperature (the boiling point).
Instead: As steam condenses it stays at 100 °C, transferring energy out as particle PE falls; only afterwards does the liquid water cool.
Mistake: Saying a solid takes the shape of its container.
Only liquids and gases do that.
Instead: Solids keep their own shape: fixed positions mean no flowing.
Mistake: Explaining compression as "particles shrinking".
Particle size never changes.
Instead: Compression closes the gaps between particles — huge in gases, almost none in solids.
Check yourself
Quick quiz
20 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Compare the physical properties of solids, liquids and gases.
Which state of matter has both a fixed shape and a fixed volume?
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Practise
Topical questions
Exam-style practice in three tiers, from recall to data questions.
- 4Foundation
- 4Standard
- 4Challenging
12 questions worth 50 marks, each with a model answer, mark scheme and examiner tips.
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Practise this chapter
Jump straight to the questions for this chapter, the chapters either side of it, and this year’s assessment papers.
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Full papers for this subject. Each school sets its own scope — check your assessment outline.
Exam practice
Practice paper
A 30-mark paper set out like a school paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 7 Practice Paper
30 marks · 40 min · 4 pages
5 multiple-choice questions and structured questions with answer lines. Chapter 7 (comparing solids, liquids and gases; arrangement, motion, forces and distances between particles; temperature and average kinetic energy; internal energy; melting, solidification, boiling and condensation at constant temperature).
Mark scheme
PROChapter 7 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Where this chapter is tested
Each school sets its own scope — check your assessment outline.
Questions
Frequently asked questions
What are the four things to mention when describing a state with the particle model?
Arrangement (regular or irregular), separation (touching or far apart), motion (vibration, sliding, or fast random movement) and the forces between the particles.
Why does the temperature stay constant during melting or boiling?
The energy transferred in goes into particle potential energy (pulling particles apart), not into their average kinetic energy — and temperature measures average kinetic energy.
Two objects are at the same temperature. Do they have the same internal energy?
No. Temperature gives the same average kinetic energy per particle, but internal energy is the total over all particles — the bigger object has more particles and so more internal energy.
How do I read a heating curve?
Sloping sections: temperature rising (single state warming). Flat sections: change of state at constant temperature — the first flat is melting, the second is boiling.