Module 4 of 9Lesson 1 of 22 · C01 States of matter and the kinetic particle theory
BGCSE Science Double Award · Chemistry · Form 4 · Section 6 Matter
Lesson C01 · about 10 minutes

States of matter and the kinetic particle theory

At a braai, the ice in the cooler box melts into water. On the fire, the water in the pot boils, and steam rises from the lid. Ice, water and steam are the same substance in three states. One simple idea explains all of it: everything is made of tiny particles that are always moving.

Prof KitsoWhen a question says “compare”, the examiners want both sides: say what the particles do in the solid and in the liquid, using words like closer, faster and stronger.
ice: solid steam: gas boiling water one substance, three states
Papers 1 and 3The same particle model is used in physics🔬 Particle model lab

Learn5 cards

Three states: how the particles are arranged

All matter is made of tiny particles. In a solid they are packed close together in a regular pattern.

In a liquid they are still close together, but irregular. In a gas they are far apart and spread out randomly.

solidliquidgas close,regular close,irregular far apart,random
Arrangement = where the particles are.

How the particles move

Solid: particles only vibrate about fixed positions, so a solid keeps its shape.

Liquid: particles slide past each other, so a liquid flows.

Gas: particles move fast and randomly in all directions.

The between particles are strongest in a solid and weakest in a gas.

solidliquidgas vibrate infixed places slide pasteach other fast, in alldirections strongweakervery weak
Movement = what the particles do. Bottom row: forces of attraction.

Changes of state

Heating causes (solid → liquid) and or (liquid → gas). Cooling causes (gas → liquid) and freezing (liquid → solid).

Some solids change straight into a gas: . Iodine and dry ice (solid carbon dioxide) do this.

solidliquidgas melting freezing boiling,evaporation condensation sublimation red: heat taken inblue: heat given out
Red arrows take in heat; blue arrows give it out.

The kinetic particle theory of melting and boiling

Heating gives the particles more . They vibrate or move faster.

At the melting point they have enough energy to overcome some forces of attraction and leave their fixed positions. At the boiling point they overcome the rest and escape as a gas.

While a substance melts or boils, its temperature stays the same.

0100 meltingboiling icewatersteam time of heating temperature / °C
Flat parts: the energy is used to overcome forces, not to raise the temperature.

Evaporation, and particles that keep their size

happens at any temperature, only at the surface: the fastest particles escape. Boiling happens all through the liquid, at the boiling point.

Particles never change size. When a substance is heated, the particles move faster and the spaces between them get bigger.

coolhot same particles, same size:only the gaps get bigger
Matter expands because the gaps grow, not the particles.

See it live

Drag the heat slider and watch the particles. Find the exact moment a solid starts to melt, then keep heating until it boils, and watch the heating curve draw itself. Then cool it back down. Finally, try iodine or dry ice: they skip the liquid state.

Particle model: heat it, cool itSolid, liquid and gas particles; melting, boiling, evaporation, condensing, freezing and sublimation, with a live heating curve

Worked example3 examples

Example 1Grade C
Use the melting and boiling points to give the state of each substance at room temperature (25 °C).
bromine: melting point −7 °C, boiling point 59 °C
sulphur: melting point 115 °C, boiling point 445 °C
oxygen: melting point −218 °C, boiling point −183 °C
rulebelow the melting point → solid; between the melting and boiling points → liquid; above the boiling point → gas
bromine25 °C is between −7 °C and 59 °C → liquid
sulphur25 °C is below 115 °C → solid
oxygen25 °C is above −183 °C → gas
One state onlyA metal melts at 29 °C and boils at 669 °C. At 50 °C it is a liquid. Do not write “it has melted” or “solid” — name the one state it is in now.
Example 2Grade B
Use the kinetic particle theory to explain what happens to the particles when ice at −10 °C is heated until it has all melted. [4]
heatingThe particles gain kinetic energy and vibrate faster about their fixed positions, so the temperature rises.
at 0 °CThe particles have enough energy to overcome some of the forces of attraction between them.
meltingThey leave their fixed positions and slide past each other: the ice becomes liquid water.
answerWhile it melts, the temperature stays at 0 °C, because the energy is used to overcome the forces of attraction.
Mark-earning words“gain kinetic energy”, “vibrate faster”, “overcome the forces of attraction”. Write about the particles, not about the ice “getting soft”.
Example 3Grade A
Solid naphthalene is heated at a steady rate. The graph shows its temperature.
051015202530050100150200250time / mintemperature / °CPQ
Heating curve for naphthalene.

(a) Give the melting point and the boiling point of naphthalene.

melting pointflat part P → 80 °C
boiling pointflat part Q → 218 °C

(b) State the physical state of naphthalene at 150 °C.

answerbetween 80 °C and 218 °C → liquid

(c) Explain why the temperature does not change during part P.

answerThe energy taken in is used to overcome the forces of attraction between the particles, not to make them move faster, so the temperature stays the same.

(d) Suggest why part Q lasts longer than part P.

answerBoiling separates the particles completely, so all the remaining forces of attraction must be overcome. This needs more energy than melting, where the particles stay close together.
Reading the timePart P lasts from 4 to 10 minutes: 6 minutes. Part Q lasts from 16 to 26 minutes: 10 minutes. Same heater, so Q took in more energy.

Misconception busterfrom the BEC examiner reports

“When it is heated, the particles expand.”
Particles never change size. They move faster and the spaces between them get bigger, so the substance expands.

Examiner report: 2023 Paper 3, Q2(d)

“Particles vibrate about fixed positions.” (no state named, no comparison)
Name the state and compare: “In the solid the particles vibrate about fixed positions; in the liquid they slide past each other.”

Examiner report: 2023 Physics (0571) Paper 3, Q4(a)(ii)

Gas particles described as “loosely packed”, or arrangement and movement mixed up.
Arrangement: far apart, random. Movement: fast, random, in all directions. Keep the two separate.

Examiner report: 2025 General Science (1430) Paper 1, Q8(d)

Melting point 29 °C, boiling point 669 °C: at 50 °C it “is a solid” or “has melted”.
Between its melting point and boiling point a substance is a liquid. State the one state.

Examiner report: 2025 General Science (1430) Paper 1, Q8(b)(i)

Prof KitsoTwo questions to ask before you write: am I describing the arrangement or the movement? And have I covered both states the question names?

Quick check5 questions · Paper 1 style

1Which describes the particles in a liquid?

  1. ✗ Not this one. That is a solid.
  2. ✗ Not this one. That is a gas.
  3. ✓ Right. Liquid particles touch but are irregular, and can move past each other — so liquids flow.
  4. ✗ Not this one. A regular pattern belongs to a solid; liquid particles are irregular.

2Solid iodine is warmed and turns straight into a purple gas. What is this change called?

  1. ✓ Right. Solid → gas with no liquid in between is sublimation.
  2. ✗ Not this one. Evaporation starts from a liquid, not a solid.
  3. ✗ Not this one. Melting gives a liquid, not a gas.
  4. ✗ Not this one. Condensation is gas → liquid, the opposite direction.

3Ice is heated. Why does the temperature stay at 0 °C until all the ice has melted?

  1. ✗ Not this one. The heater still supplies energy; the energy goes somewhere else.
  2. ✗ Not this one. Particles never change size.
  3. ✗ Not this one. Heat does pass into the ice — that is why it melts.
  4. ✓ Right. Energy breaks the forces holding particles in place, so they do not speed up.

4Steam hits a cold pot lid and forms drops of water. What happens to the particles?

  1. ✗ Not this one. Gaining energy would keep them a gas; condensing means losing energy.
  2. ✓ Right. Condensation: particles lose kinetic energy and the forces of attraction pull them close together.
  3. ✗ Not this one. Particles never change size.
  4. ✗ Not this one. Particles in a liquid still move; they slide past each other.

5Mercury melts at −39 °C and boils at 357 °C. What is its state at 25 °C?

  1. ✗ Not this one. 25 °C is above its melting point, so it is no longer solid.
  2. ✗ Not this one. 25 °C is far below its boiling point.
  3. ✓ Right. 25 °C is between −39 °C and 357 °C, so mercury is a liquid.
  4. ✗ Not this one. Two states exist together only at the melting point itself.

Exam cornerPaper 3 style · 8 marks

Fig. 1.1 shows the particles of one substance in three different states, A, B and C.

ABC
Fig. 1.1
(a)
Which diagram, A, B or C, shows the substance as a gas? [1]
(b)
Describe the arrangement and the movement of the particles in B. [2]
(c)
Name the change of state from C to A. [1]
(d)
Use the kinetic particle theory to explain how A changes into C when it is cooled. [3]
(e)
Dry ice is solid carbon dioxide. At −78 °C it changes straight into a gas. Name this change. [1]
Model answer

(a)

A

(b)

Arrangement: the particles are close together in a regular pattern.
Movement: they vibrate about fixed positions.

(c)

Boiling (or evaporation).

(d)

The particles lose kinetic energy, so they move more slowly.
The forces of attraction pull them close together, so the gas condenses into a liquid.

(e)

Sublimation.
Marking key — how the examiner awards the marks
Mark pointMark
(a)AB1
(b)arrangement: close together / touching in a regular patternB1
movement: vibrate about fixed positionsB1
(c)boiling / evaporationB1
(d)particles lose (kinetic) energyB1
move more slowlyB1
forces of attraction pull them close together (but not in fixed positions)B1
(e)sublimationB1

Total: 8 marks

Examiner tip. In (b), give arrangement and movement on separate lines so each mark point is easy to see. In (d), write about energy, speed and forces of attraction — three ideas for three marks.

Rememberone-page card to download

  • Solid: close, regular, vibrate about fixed positions. Liquid: close, irregular, slide past each other. Gas: far apart, random, move fast in all directions.
  • Forces of attraction: strong in solids, weaker in liquids, very weak in gases.
  • Heating → particles gain kinetic energy → vibrate or move faster → overcome forces of attraction.
  • The temperature stays the same while a substance melts or boils.
  • Evaporation: any temperature, surface only. Boiling: at the boiling point, all through the liquid.
  • Particles never change size; the spaces between them change.

Changes of state

meltingsolid → liquid (heat in)
freezingliquid → solid (heat out)
boiling, evaporationliquid → gas (heat in)
condensationgas → liquid (heat out)
sublimationsolid → gas (iodine, dry ice)
Download this card (PDF)

Science that earnsa skill you can sell

Ice blocks for braais and events

On hot weekends, people at braais, weddings and football matches want cold drinks. Bags of ice sell well, and you can make them in a home freezer.

The science helps you sell: melting ice takes in heat from the drinks and stays at 0 °C until it has all melted. Big blocks melt more slowly than small cubes, so they last the whole afternoon.

Example costs
40 strong ice bags and tiesP30
Clean drinking waterP10
Extra electricity for the freezer (a week)P50
Costs for 40 bagsP90
Sales: 40 bags at P8P320
ProfitP230

Ice that touches drinks is food: use only clean drinking water, wash your hands, and keep the bags sealed and frozen until sold. Ask the owner of the freezer first, and sell from a cooler box so the ice does not melt in the Sun’s radiation.

Example figures only — check real prices and ask your council whether you need a food-hawker permit.

Syllabus checklist Tap when you can do it

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Stadira Training Institute · Gaborone, Botswana · BGCSE Science Double Award (0569) Complete Tutor

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