Overview
What you’ll learn
- Describe the arrangement, separation, motion and forces between particles in solids, liquids and gases.
- Relate the physical properties of each state (shape, volume, compressibility) to its particle model.
- Explain melting, boiling, condensation, freezing and sublimation in terms of particle energy, separation and forces of attraction.
- Interpret heating and cooling curves, identifying what happens on the sloping parts and the flat plateaus.
- Explain diffusion in gases and liquids as evidence for moving particles.
- Predict how relative molecular mass and temperature change the rate of diffusion.
In 30 seconds
- 1All matter is made of tiny particles in constant random motion; the state of a substance depends on the balance between particle energy and forces of attraction.
- 2Solid: particles touch, vibrate about fixed positions, ordered lattice — fixed shape and volume, not compressible.
- 3Liquid: particles touch but can slide past one another — fixed volume, takes the shape of the container, almost incompressible.
- 4Gas: particles are far apart, move fast and randomly — fills and takes the shape of the container, easily compressed.
- 5During a change of state the temperature stays constant: energy goes into overcoming forces of attraction (or is released forming them).
- 6On a heating curve, sloping parts mean the particles are heating up; flat plateaus mean the state is changing.
- 7Diffusion is the net movement of particles from where they are more concentrated to where they are less concentrated.
- 8Diffusion is faster at higher temperatures and faster for lighter particles (smaller relative molecular mass).
Notes · 1 of 5
Three states, one model
Same particles, different arrangements: the state is decided by energy versus attraction.
The kinetic particle theory says all matter is built from particles that are always moving. How tightly they are held decides the state. In a solid, each particle vibrates about a fixed point in an ordered lattice — the attractions are strong enough to lock neighbours in place. In a liquid, the particles still touch but have enough energy to shuffle past one another. In a gas, the particles have escaped each other’s pull almost entirely and fly about at high speed.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | Regular, close-packed lattice | Clustered but disordered | Far apart, random |
| Motion | Vibrate about fixed positions | Slide and tumble past neighbours | Fast, straight-line, random |
| Forces between particles | Very strong | Moderate | Negligible |
| Shape | Fixed | Takes the container’s shape | Fills the container |
| Volume | Fixed | Fixed | Spreads to fill any volume |
| Compressibility | Almost none | Almost none | High — gaps between particles |
The compressibility fact is a favourite exam check: squeezing a gas pushes particles into the empty space between them, but a solid or liquid has no such gaps, so its volume barely changes. Shape and volume answers must be explained with the particle picture — "gas particles are far apart and move in all directions" earns the mark; "gases are light" does not.
Notes · 2 of 5
Changes of state as energy stories
Melting, boiling, condensing, freezing and sublimation — each is energy overcoming or releasing attraction.
| Change | Direction | Energy | Particle explanation |
|---|---|---|---|
| Melting | Solid → liquid | Taken in | Particles gain energy and vibrate so hard the lattice breaks; attractions weaken |
| Freezing | Liquid → solid | Given out | Slowing particles lock into fixed positions; attractions pull them into the lattice |
| Boiling | Liquid → gas | Taken in | Particles gain enough energy to break away from each other completely, forming bubbles through the liquid |
| Condensation | Gas → liquid | Given out | Slowing particles fall close enough for attractions to hold them together |
| Evaporation | Liquid → gas, surface only, at any temperature | Taken in | The fastest surface particles escape; the average energy of those left behind falls |
| Sublimation | Solid → gas directly | Taken in | Particles gain enough energy to leave the lattice without passing through a liquid |
Two subtle points earn marks. First, temperature measures average kinetic energy — during melting or boiling the energy absorbed goes into weakening attractions, not speeding particles up, so the thermometer holds still. Second, evaporation differs from boiling: evaporation happens only at the surface and at any temperature (puddles dry without boiling), while boiling happens throughout the liquid at one fixed temperature.
Sublimation is rarer: the syllabus favourites are iodine (grey-purple crystals to violet vapour on warming) and dry ice, solid carbon dioxide, which puffs straight to gas at room conditions. The reverse change (gas to solid) is deposition — often called "sublimating in reverse" in school answers.
Notes · 3 of 5
Reading heating and cooling curves
Slopes heat one state; plateaus change the state at constant temperature.
A heating curve plots temperature against time as a substance is heated steadily. Its shape tells the whole energy story:
- 1First slope
Solid warms: particles vibrate faster, temperature climbs.
- 2First plateau
Melting: energy goes into breaking the lattice. Temperature stays constant at the melting point until every gram is liquid.
- 3Middle slope
Liquid warms: particles slide faster.
- 4Second plateau
Boiling: energy goes into separating particles completely. Temperature holds at the boiling point until the last of the liquid is gone.
- 5Final slope
Gas warms: particles fly faster still.
Cooling curves are the mirror image: slopes cool, plateaus freeze or condense. A plateau that sags slightly instead of staying flat is the fingerprint of an impure sample — remember Chapter 1: impurities spread the change of state over a range.
Notes · 4 of 5
Diffusion in gases
Particles spread from high to low concentration — faster when light and hot.
Diffusion is the spreading of particles through random motion from a region of higher concentration to one of lower concentration. It is direct evidence that particles move: drop bromine vapour into an evacuated tube and it fills it instantly; into air and it still spreads, but slowly — because the bromine particles keep colliding with air particles on the way.
Two rate rules cover every question: diffusion is faster at higher temperature (particles move faster on average) and faster for smaller relative molecular mass (at the same temperature, lighter particles move faster than heavy ones). Both must be explained in particle language — "the particles have more kinetic energy" or "the particles have less mass, so they move faster".
Notes · 5 of 5
Diffusion in liquids and why temperature matters
Same spreading, much slower — and a temperature rise speeds it in both states.
A crystal of potassium manganate(VII) dropped into still water colours its corner deep purple; days later the whole beaker is evenly coloured. Diffusion in liquids is far slower than in gases because liquid particles are packed close together — a moving particle collides with neighbours almost constantly, so its journey is a drunken crawl rather than a flight.
Warming the water visibly speeds the spreading: hotter particles move faster, collide more, and carry the dissolved substance outward sooner. Stirring is not diffusion — it is bulk mixing — but it helps the same way by sweeping fresh solvent over the crystal.
Model check
What diffusion proves
Matter is particles, in constant random motion — otherwise colour would stay put.
Model check
What speed reveals
Comparing diffusion rates of different gases reveals their relative molecular masses.
Model check
What it cannot do
Diffusion cannot separate a pure substance — it mixes; separation needs Chapter 1 techniques.
Revise
Key terms
The words examiners expect you to use precisely.
- Kinetic particle theory
- The model that all matter consists of tiny particles in constant random motion.
- Melting point
- The fixed temperature at which a solid becomes a liquid; the lattice breaks down.
- Boiling point
- The fixed temperature at which a liquid boils throughout, becoming gas.
- Sublimation
- A change of state from solid directly to gas, e.g. iodine or dry ice.
- Latent heat
- Energy absorbed or released during a change of state at constant temperature.
- Evaporation
- Slow change from liquid to gas at the surface only, at any temperature.
Show 4 more key termsShow fewer key terms
- Diffusion
- Net spreading of particles from a region of higher to lower concentration.
- Relative molecular mass (Mr)
- The sum of the relative atomic masses in a molecule; lighter molecules diffuse faster.
- Plateau
- The flat section of a heating or cooling curve where the state changes at constant temperature.
- Compressibility
- How much a substance’s volume shrinks under pressure; high for gases only.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Saying particles expand when a substance is heated.
Particle size never changes; solids and liquids expand because particles vibrate further apart, gases because they fly apart.
Instead: Heating increases particle speed and separation, never particle size.
Mistake: Saying temperature rises during melting because "heat is still added".
On the plateau the added energy breaks attractions instead of raising average kinetic energy.
Instead: During a change of state, temperature stays constant while latent heat is absorbed.
Show 10 more mistakesShow fewer mistakes
Mistake: Using "heat" and "temperature" interchangeably.
Heat is energy transferred; temperature measures the average kinetic energy of particles.
Instead: A sparkler spark has a higher temperature but transfers less heat than a bath of warm water.
Mistake: Claiming gases are compressible because "gas particles are small".
Solid and liquid particles are just as small but their states barely compress.
Instead: Gases compress because of the large empty spaces between particles.
Mistake: Writing that boiling happens only at the surface.
That describes evaporation; boiling forms bubbles of vapour throughout the liquid.
Instead: Boiling happens throughout the liquid at one fixed temperature; evaporation is surface-only at any temperature.
Mistake: Explaining evaporation cooling as "cold particles come in".
Nothing cold arrives — the fastest particles leave, lowering the average energy of those remaining.
Instead: Higher-energy surface particles escape; the average kinetic energy of the rest falls, so the temperature drops.
Mistake: Saying the white ring forms exactly halfway in the ammonia–HCl tube.
Ammonia (Mr 17) moves faster than hydrogen chloride (Mr 36.5), so it travels further before meeting.
Instead: The ring forms closer to the HCl end because the lighter ammonia covers more distance per second.
Mistake: Explaining faster hot diffusion as "hot particles are bigger".
Size is unchanged; temperature changes particle speed.
Instead: At higher temperature the average kinetic energy is higher, so particles spread faster.
Mistake: Reading a cooling-curve plateau as "nothing is happening".
The substance is freezing — losing energy while attractions lock particles into the lattice.
Instead: Plateau = change of state at constant temperature; energy is still leaving.
Mistake: Confusing the plateau order on a heating curve.
Students often put the shorter melting plateau above the boiling plateau.
Instead: Melting plateau comes first (lower temperature), and boiling’s plateau is longer — vaporising needs more energy.
Mistake: Saying a liquid takes the shape of its container because its particles shrink.
Shape change is about mobility, not particle size.
Instead: Liquid particles can slide past one another while remaining in contact.
Mistake: Treating stirring as diffusion in an experiment write-up.
Stirring is bulk movement of the liquid, not random particle motion.
Instead: Diffusion is the random-motion spreading; stirring is a separate aid to mixing.
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
Describe particle arrangements
In which state of matter are the particles furthest apart?
Ready for more practice on Chapter 2: Kinetic Particle Theory?
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Practise
Topical questions
Exam-style practice in three tiers, from recall to data questions.
- 4Foundation
- 5Standard
- 3Challenging
12 questions worth 48 marks, each with a model answer, mark scheme and examiner tips.
Practise topical questionsExam 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 2 Practice Paper
30 marks · 40 min · 4 pages
5 multiple-choice questions and structured questions with answer lines. Chapter 2 (states of matter, changes of state, heating and cooling curves, diffusion).
Mark scheme
PROChapter 2 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 does Sec 3 Chemistry Chapter 2 cover?
It covers sub-topic 2.1 of O-Level Pure Chemistry 6092: the kinetic particle theory, the arrangement and motion of particles in solids, liquids and gases, all the changes of state with energy explanations, heating and cooling curves, and diffusion in gases and liquids with its rate factors.
Why does the temperature stay constant while something melts or boils?
Temperature measures the average kinetic energy of the particles. On a melting or boiling plateau the energy you add goes into weakening or breaking the attractions between particles instead of speeding them up, so the average kinetic energy — and therefore the thermometer reading — stays fixed until the change of state is complete.
How do I explain diffusion answers to get full marks?
Always answer in particle language: particles move in random motion, collide with the particles already there, and spread from where they are more concentrated to where they are less concentrated. Then add the rate factor asked: hotter means faster particles, and lighter (smaller Mr) means faster particles.
Which gases should I know sublime?
Iodine (grey-purple crystals giving violet vapour) and solid carbon dioxide (dry ice) are the two the syllabus expects. Both turn from solid straight to gas without melting when warmed at ordinary pressure.
When is this chapter usually tested in school?
Kinetic particle theory sits early in Sec 3, so it appears in Weighted Assessment 1 (Term 1) and again in the end-of-year examination — and its particle explanations keep earning marks inside later topics such as rate of reactions and electrolysis. Scope varies by school, so check your school’s assessment outline.
Up next
Chapter 3: Atomic Structure