Solids, liquids and gases: the particle model
By the end of this lesson you will be able to
- Describe the properties of solids, liquids and gases — shape, volume, flow, compressibility, density — and explain each using the arrangement and movement of particles.
- Draw particle diagrams of the three states that an examiner would accept.
- Name the changes of state — melting, freezing, boiling, evaporation, condensation, sublimation — and explain the relationship between energy and state.
- Interpret a heating curve and explain why the temperature stays constant while a substance melts or boils.
- Explain the effects of heating and cooling on materials such as metals (expansion and contraction) and their consequences in everyday life.
The puzzle
Put a beaker of crushed ice on a flame with a thermometer in it. The reading climbs to 0 °C and then stops — for minutes — while the flame roars underneath and the ice slowly turns to water. The flame is pouring energy into the beaker the whole time. Where is it going, if not into making the thermometer rise?
A bottle of perfume is opened at one side of a room; a minute later someone at the far side can smell it. Nobody blew it across. Write down what you think must be true about the stuff a smell is made of for that to happen.
Compare your answer
The perfume must be made of pieces far too small to see, which leave the liquid, move on their own through the air in every direction at speed, and spread out until they reach every corner. That is the whole particle model in one observation: matter is made of tiny particles; they are always moving; the hotter they are, the faster they move; and in a gas they are far apart with nothing between them. Every property of solids, liquids and gases in this lesson follows from those four sentences.
The science — structured notes
1.1 The particle model
All matter is made of particles — atoms, or groups of atoms — far too small to see: a drop of water holds more of them than there are grains of sand on every beach on Earth. The particle model (also called kinetic theory) says:
1.2 The three states and their properties
| Solid | Liquid | Gas | |
|---|---|---|---|
| Arrangement | Regular pattern, touching | Random, touching, no pattern | Random, far apart |
| Movement | Vibrate about fixed positions | Move past one another, slowly | Move fast and freely in all directions |
| Forces between particles | Strong | Weaker — enough to hold them together, not to fix them | Almost none |
| Shape | Fixed | Takes the shape of its container | Fills its container |
| Volume | Fixed | Fixed | Fills whatever space it is given |
| Can it flow? | No | Yes | Yes |
| Can it be compressed? | No — particles already touching | Hardly — particles touching | Yes — large spaces between particles |
| Density | High | High (a little lower than the solid, usually) | Very low — about a thousandth of the liquid |
Explain why a gas can be compressed but a liquid cannot.
In a gas the particles are far apart with empty space between them, so pushing the piston moves the particles closer together into that space.
In a liquid the particles are already touching; there is no empty space to squeeze out, and pushing them closer would need enormous force against the repulsion of the particles.
Two states, one difference (spacing), one consequence each. Naming the state without the particles (“gases are squashy”) earns nothing.
Use the particle model to explain each.
(a) A solid keeps its shape when you pick it up, but a liquid runs through your fingers.
(b) One litre of water becomes about 1700 litres of steam.
(c) A bicycle tyre becomes harder to pump as it fills.
Reveal answer
(a) In a solid the particles are held in fixed positions by strong forces and only vibrate, so the arrangement — and the shape — cannot change. In a liquid the forces are weaker and the particles can slide past one another, so the liquid changes shape and flows, while the particles stay touching (fixed volume).
(b) The same number of particles, but in the gas they are about ten times further apart in every direction, and the space between them is empty; a small volume of liquid becomes a large volume of gas. The particles themselves have not grown.
(c) Air is a gas; pumping forces more particles into the same space and pushes them closer together. The more particles there are hitting the inside of the tyre each second, the greater the pressure pushing back on the pump.
1.3 Changes of state — energy in, energy out
| Change | From → to | Energy | What happens to the particles |
|---|---|---|---|
| Melting | Solid → liquid | Taken in | Vibrate harder until the forces can no longer hold them in place; they break free and slide |
| Freezing (solidifying) | Liquid → solid | Given out | Slow down until the forces pull them into fixed positions |
| Boiling | Liquid → gas, throughout the liquid, at the boiling point | Taken in | Move fast enough to escape the forces completely; bubbles of gas form inside the liquid |
| Evaporation | Liquid → gas, from the surface only, at any temperature | Taken in | The fastest particles at the surface escape; the rest are left slower, so the liquid cools |
| Condensation | Gas → liquid | Given out | Particles slow down enough for the forces to pull them together |
| Sublimation | Solid → gas directly | Taken in | Dry ice (solid carbon dioxide) and iodine do this; frost disappearing on a dry morning is water doing it |
A solid is heated steadily. Temperature readings: 20, 35, 50, 50, 50, 50, 62, 74 °C at one-minute intervals.
Minutes 0–2: temperature rising — the solid is warming; particles vibrate faster.
Minutes 2–5: constant at 50 °C — the solid is melting; the melting point is 50 °C; energy is breaking the forces between particles, not raising their speed.
Minutes 5–7: rising again — the liquid is warming. A pure substance gives a perfectly flat step; a mixture gives a slope (Lesson 6.2).
A puddle disappears on a warm day although the water never reached 100 °C. Explain.
The particles in a liquid do not all have the same energy; some at the surface are moving fast enough to escape the forces of the others. Those leave as gas — evaporation happens from the surface at any temperature.
Boiling is different: at 100 °C particles throughout the liquid have enough energy to form bubbles of gas. Wind, warmth and a large surface speed up evaporation, which is why washing dries faster on a hot, windy day spread out on the line.
Because the fastest particles leave, the average energy of those left falls — evaporation cools the liquid. Sweat cools you; a clay pot keeps water cool; a wet cloth cools a fevered forehead.
“Water boils faster if you turn the flame up, so it must get hotter than 100 °C.”
Reveal answer
False. At sea level liquid water cannot be hotter than 100 °C; extra energy makes it boil faster, turning liquid into steam faster, not hotter. Every joule goes into separating particles. (On the Botswana plateau at about 1000 m the boiling point is around 97 °C, because the air pressure is lower — one reason tea tastes different at altitude and rice takes longer to cook.)
1.4 Heating and cooling materials: expansion and contraction
Heating a solid does not only make it warmer; it makes it slightly bigger. The particles vibrate more violently and take up more room, so the whole object expands; cooling lets them settle closer and it contracts. The change is small — a 100 m steel bridge grows about 6 cm between a cold night and a hot afternoon — but the forces involved are enormous, and engineers must allow for them.
| Where it matters | What is done about it |
|---|---|
| Railway lines and bridges | Expansion gaps or sliding joints so that hot rails cannot buckle; long welded rails are pre-stretched |
| Overhead power lines | Hung with slack so that they can sag in summer heat and tighten in winter without snapping |
| Concrete roads and pavements | Joints filled with tar that squashes as slabs expand |
| A jar lid that will not open | Run hot water over the metal lid — it expands more than the glass and loosens |
| Thermometers | The liquid expands up a thin tube; the expansion is the measurement |
| Thermostats | A strip of two metals that expand by different amounts bends as it warms and switches a circuit off — kettles, irons, ovens |
| Fitting a metal tyre to a wagon wheel | Heat the tyre so it expands, slip it on, let it cool and grip |
A strip of brass bonded to a strip of steel is straight at room temperature. Explain why it bends when heated, and how that can switch off a kettle.
Both metals expand when heated, but brass expands more than steel for the same rise in temperature.
Because they are bonded, the strip cannot stay straight: the brass side becomes longer than the steel side, so the strip curves with brass on the outside of the bend.
In a kettle the bent strip pushes a contact apart and breaks the circuit when the water reaches boiling; as it cools, it straightens and the contact closes again. Expansion, a nuisance on a bridge, is the working part of a switch.
Try it — the heating curve
Heat 100 g of crushed ice with a 40 W heater and read the temperature every minute from 0 to 16 minutes. Plot temperature against time: find the plateau and read the melting point. Then double the mass, or halve the heater power, and see what happens to the length of the plateau. Finally switch to stearic acid and check whether its plateau sits at 69 °C.
Suppose your readings for ice were: 0 min → −24 °C, 1 → −1, 2 → 0, 3 → 0, 4 → 0, 5 → 0, 6 → 0, 7 → 0, 8 → 0, 9 → 11, 10 → 23, 11 → 34, 12 → 46 °C.
(a) Describe the graph in three parts, quoting values, and state the melting point.
(b) Explain, using the particle model, why the temperature stays at 0 °C from about 2 to 8 minutes.
(c) Predict what changing the mass to 200 g would do to each part of the graph, with a reason.
Reveal answer
(a) From 0 to about 2 minutes the temperature rises quickly from −24 °C to 0 °C (the solid warming); from about 2 to 8 minutes it stays at 0 °C (the plateau — melting); from 8 to 12 minutes it rises again at about 11 °C per minute (the liquid warming). The melting point is 0 °C, read from the plateau.
(b) At 0 °C the energy from the heater is used to break the forces holding the water particles in the fixed positions of the ice crystal, freeing them to slide past one another. It does not increase how fast the particles move, and temperature measures speed of movement, so the thermometer does not rise until all the ice has melted.
(c) Twice the mass means twice as many particles to warm and to free with the same energy per minute: the first rise would take twice as long (about 4 minutes), the plateau would be twice as long (about 12 minutes, so it would not end within the 16-minute run), and the liquid would warm at half the rate. The plateau temperature would not change — the melting point is a property of the substance, not the amount.
Common mistakes to avoid
- Particles that melt, expand or change size — particles do not melt; the arrangement changes. Particles do not get bigger when heated; they move more and spread out.
- “Air between the particles” — between particles there is nothing.
- Gas particles drawn at the bottom of the box — they fill the whole space.
- “The temperature stops rising because the heat is used up” — energy is being taken in the whole time; it is breaking forces, not raising speed.
- Evaporation = boiling — evaporation is from the surface at any temperature; boiling is throughout the liquid at the boiling point.
- Explaining without the particles — “gases are compressible because they are gases” earns nothing; “because the particles are far apart with empty space between them” earns the mark.
How we found out — the pollen that would not keep still
The idea that everything is made of tiny indivisible pieces is 2400 years old: the Greek philosopher Democritus argued that if you cut a thing in half again and again you must reach something that cannot be cut — atomos. It was an argument, not an experiment, and for two thousand years it stayed one. In 1808 the English schoolteacher John Dalton turned atoms into chemistry: elements are made of atoms, atoms of different elements have different masses, and compounds are atoms joined in fixed ratios. He weighed gases and compounds to prove it. But an atom was still something nobody could see or point to, and serious scientists as late as 1900 called it a useful fiction.
The clue that atoms were real had been sitting in a microscope since 1827. That year the botanist Robert Brown — the man who named the nucleus — watched pollen grains suspended in water and saw them jiggling endlessly, as if alive. Dead pollen jiggled too; so did dust and soot. He could not explain it. In 1905 Albert Einstein, then a patent clerk, worked out that the jiggling is exactly what you would expect if the grain is being hit from all sides, at random, by water particles far too small to see — and he predicted how far a grain should wander in a given time depending on the size of the water particles. In 1908 the French physicist Jean Perrin measured the wandering under a microscope, and the numbers matched. Atoms had been counted by the way they jostled something visible. Perrin received the Nobel Prize in 1926; the last serious doubters gave up. Brownian motion is still the simplest evidence for the particle model — you can watch it yourself with smoke in a glass cell under a school microscope.
Why did Brownian motion convince scientists of the existence of particles when Dalton’s careful weighings had not?
Reveal answer
Dalton’s ratios could be explained without atoms — as a rule about masses — so the evidence was consistent with atoms but did not require them. Brownian motion is a visible effect of the particles themselves: something is hitting the pollen grain, at random, from every side, and only invisible moving particles explain it. Einstein turned that into a numerical prediction (how far a grain wanders per second, and how that depends on temperature and particle size), and Perrin’s measurement matched. A prediction that could have failed, and did not, is what turns a model into accepted science.
Where it is used now
- The Makgadikgadi and Sua Pans. A shallow lake evaporates in Botswana’s sun, leaving salt and soda; the pans are evaporation on a scale you can see from space, and Lesson 6.4 shows how Botswana Ash turns it into an industry.
- Refrigerators and air conditioners. A liquid evaporates inside the fridge, taking in energy (cooling the food), and is squeezed back into a liquid outside, giving the energy out at the back. Every fridge is a changes-of-state machine.
- Cooking gas. LPG is stored as a liquid under pressure in the cylinder; when the valve opens the pressure drops and it boils to a gas, which is why the cylinder gets cold and frosty in use.
- Weather. Clouds are condensation; rain is water that fell as droplets grew; dew on a cold morning is the air’s water vapour condensing on grass that lost its energy overnight. The water cycle is evaporation and condensation on a planetary scale.
- Engineering. Expansion joints in every bridge and railway; the bimetallic thermostat in every kettle and iron; concrete poured at night in hot countries so that it does not crack as it cools.
Science now — and the scientist you need to be
What is happening in 2026
The three states are the ones you meet at home; laboratories now work routinely with more. Plasma — a gas so hot that electrons are torn from atoms — is the state of stars and of the inside of the fusion reactors being built in France and tested by private companies, where the aim is to hold a plasma ten times hotter than the Sun’s core for long enough to generate power. At the other extreme, atoms cooled to a few billionths of a degree above absolute zero stop behaving as separate particles at all and form a single quantum state, used in the most precise clocks ever built. Between those extremes, materials that change state on command are becoming ordinary: phase-change materials that melt at 25 °C are cast into wall panels so that a building absorbs heat by day and releases it at night — a use of latent heat for cooling houses without electricity that is being trialled in hot climates, including southern Africa. Every one of these begins with the particle model you drew today, extended to conditions the model’s inventors never imagined.
The mindshift
What you need to be able to do
The examiner reports for every MYP Sciences session make the same complaint about chemistry answers: candidates describe what they see and never mention particles. “The gas spreads out” is an observation; “the particles move randomly at speed and there are no forces holding them together, so they spread into all the available space” is an explanation. Every answer in this unit — dissolving, separating, expanding, conducting — should end at the particle level. The habit: observation → particles (arrangement, movement, forces, spacing) → consequence.
See an example
Condensation on the outside of a cold bottle: gas → liquid, energy given out by the water vapour to the cold glass; particles slow and are pulled together. Washing drying: liquid → gas from the surface at 30 °C, energy taken in from the sun and air; the fastest surface particles escape. Ice in a glass: solid → liquid, energy taken in from the drink (which is why the drink cools); particles break free of the crystal pattern. The AI said sweat “evaporates and takes heat away”; when I asked why that cools the skin it added that the highest-energy particles leave first, lowering the average — which is the full answer.
Key terms
| Particle model | All matter is made of tiny particles that are always moving, attract one another, and move faster when hotter; between them is empty space. |
| Solid / liquid / gas | The three states: particles fixed and vibrating; touching and sliding; far apart and moving freely. |
| Melting / freezing | Solid → liquid (energy taken in) / liquid → solid (energy given out); both at the melting point. |
| Boiling | Liquid → gas throughout the liquid at the boiling point. |
| Evaporation | Liquid → gas from the surface at any temperature; it cools the liquid left behind. |
| Condensation | Gas → liquid; energy given out. |
| Sublimation | Solid → gas directly, without melting. |
| Heating curve | A graph of temperature against time for a substance heated steadily; flat steps mark melting and boiling. |
| Latent heat | The energy taken in or given out during a change of state without a change in temperature. |
| Expansion / contraction | The increase / decrease in size of a material when heated / cooled, because its particles vibrate more / less. |
| Bimetallic strip | Two metals bonded together that bend when heated because they expand by different amounts; used as a switch. |
Think like a scientist
Criterion A A cook in Maun puts a pot of water on a gas flame at 8:00. At 8:04 the water is at 60 °C; at 8:07 it is boiling at 97 °C; at 8:20 it is still at 97 °C but the pot is half empty. She lifts the lid and drops of water run off the underside. Later she leaves the empty steel pot in the sun and cannot get its tight-fitting lid off until she runs cold water over the pot.
(a) Explain, using the particle model, why the temperature rose from 8:00 to 8:07 but not from 8:07 to 8:20, even though the flame was the same.
(b) Explain why the water boils at 97 °C in Maun rather than 100 °C.
(c) Explain what the drops under the lid are and where the energy for the change came from.
(d) Explain why the pot was half empty, distinguishing this from evaporation.
(e) Explain why the lid came off after cold water was run over the pot, and suggest why heating the lid alone would also have worked.
Model answer
(a) From 8:00 to 8:07 the energy from the flame made the water particles move faster, and temperature measures the average speed of the particles, so the reading rose. At 97 °C the water boils: from then on the energy is used to separate particles completely from the liquid — breaking the forces between them so that they escape as gas — and not to increase their speed, so the temperature stays constant however long the flame burns.
(b) Maun is about 950 m above sea level, so the air pressure is lower. Boiling happens when particles can form bubbles of gas inside the liquid against the pressure of the air above; with less pressure pushing down, the particles need less energy to do it, so the liquid boils at a lower temperature.
(c) Steam — water in the gas state — rises from the boiling water and hits the lid, which is cooler. The gas particles give energy to the lid, slow down, and the forces between them pull them back together into liquid: condensation. The energy came originally from the flame, was carried by the steam, and was given out to the lid.
(d) Half the water has boiled away: at the boiling point, particles throughout the liquid gain enough energy to escape as gas, so bubbles form everywhere in the pot and the liquid turns to steam rapidly. Evaporation would be the slow escape of only the fastest particles from the surface at any temperature — a pot left on the table also empties, but over days, not minutes.
(e) In the sun both pot and lid warmed and expanded, and the lid jammed in the rim; the pot, thicker and holding more energy, kept the lid tightly gripped. Cold water cooled the pot’s rim first, so its particles vibrated less and it contracted away from the lid, loosening the fit. Heating the lid alone would work for the same reason in reverse: the lid’s particles would vibrate more, the lid would expand outwards from the rim, and the grip would loosen.
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
Describe the arrangement and movement of particles in a solid, a liquid and a gas.
Reveal answer
Solid: regular pattern, touching, vibrating about fixed positions [1]; liquid: touching but random, moving past one another [1]; gas: far apart, random, moving fast in all directions [1].
Explain why a liquid has a fixed volume but not a fixed shape.
Reveal answer
The particles are touching and the forces between them keep them together, so the volume cannot change [1]; but the forces are not strong enough to hold them in fixed positions, so they slide past one another and the liquid takes the shape of its container [1].
A student writes: “When ice melts, the particles melt and get bigger, and the temperature rises because the heat goes into the particles.” Identify three errors.
Reveal answer
Particles do not melt — the arrangement changes as forces between particles are overcome [1]; particles do not change size [1]; the temperature does not rise during melting — the energy breaks the forces rather than speeding the particles up [1].
A steel railway line is laid in 25 m lengths with small gaps between them. Explain why the gaps are needed and what would happen without them on a hot day.
Reveal answer
When the steel is heated its particles vibrate more and take up more space, so each rail expands and becomes longer [1]; the gap gives the rail room to expand [1]; without gaps the expanding rails would push against each other with enormous force and buckle sideways, derailing trains [1].
True or false, with a reason: “Evaporation only happens when a liquid is boiling.”
Reveal answer
False [1]: evaporation happens from the surface at any temperature, because some particles at the surface always have enough energy to escape; boiling happens throughout the liquid only at the boiling point [1].
A heating curve for a substance shows two flat steps, at −7 °C and at 58 °C. State what the substance is at 20 °C and at 70 °C, and explain what is happening to the particles during the step at 58 °C.
Reveal answer
At 20 °C it is a liquid (above its melting point of −7 °C and below its boiling point of 58 °C) [1]; at 70 °C it is a gas [1]; at 58 °C the energy supplied is separating the particles completely from one another — overcoming the forces between them so they escape as gas — rather than making them move faster, so the temperature stays constant until all the liquid has boiled [1].
Summary
- Matter is made of particles that are always moving, attract one another, move faster when hotter, and have empty space between them.
- Solid: fixed pattern, vibrating. Liquid: touching, sliding. Gas: far apart, flying. Shape, volume, flow, compressibility and density all follow.
- Melting and boiling take energy in; freezing and condensing give it out. During a change of state the temperature stays constant because the energy breaks forces, not speeds.
- Evaporation is from the surface at any temperature and cools the liquid; boiling is throughout the liquid at the boiling point.
- Heating expands materials because particles vibrate more; engineers allow for it, and thermostats use it.
Next time → What the particles are: elements, compounds and mixtures — and what a chemist means by “pure”.