Overview
What you’ll learn
- Describe the distinguishing properties of solids, liquids and gases.
- Use the kinetic particle theory to describe particle arrangement, separation and motion in each state.
- Describe melting, boiling, evaporating, freezing and condensing.
- Describe diffusion and explain how temperature and particle mass affect its rate.
- Explain changes of state and the effect of pressure on a gas using the kinetic particle theory (Supplement).
Syllabus checklist (0620)
Statements are paraphrased and grouped by tier. Core is studied by everyone; Supplement is examined on the Extended papers only.
Core — every candidate
- 1.1State the distinguishing properties of solids, liquids and gases.
- 1.1Describe the structures of solids, liquids and gases in terms of particle separation, arrangement and motion.
- 1.1Describe changes of state in terms of melting, boiling, evaporating, freezing and condensing.
- 1.1Describe the effects of temperature and pressure on the volume of a gas.
- 1.2Describe and explain diffusion in terms of the kinetic particle theory.
Supplement — Extended candidates only
- 1.1Explain changes of state in terms of the kinetic particle theory, including the interpretation of heating and cooling curves.
- 1.1Explain, using the kinetic particle theory, the effects of temperature and pressure on the volume of a gas.
- 1.2Describe and explain the effect of relative molecular mass on the rate of diffusion of gases.
In 30 seconds
- 1In a solid, particles are close together in a regular arrangement and only vibrate about fixed positions.
- 2In a liquid, particles are close but randomly arranged, and can slide past one another.
- 3In a gas, particles are far apart and move quickly in all directions.
- 4Melting and boiling absorb energy; freezing and condensing release energy.
- 5Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.
- 6A lighter gas diffuses faster than a heavier one at the same temperature (Supplement).
Notes · 1 of 5
The three states of matter
Solids, liquids and gases differ in how their particles are arranged and how they move.
Every substance is made of tiny particles. The kinetic particle theory says that these particles are always moving, and that the way they are arranged and how much they move decides whether the substance is a solid, a liquid or a gas.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed | Takes the shape of the container | Fills the whole container |
| Volume | Fixed | Fixed | Fills the container; can be compressed |
| Particle separation | Very close | Close | Very far apart |
| Particle arrangement | Regular, ordered | Random, still touching | Random, widely spaced |
| Particle motion | Vibrate about fixed positions | Slide past one another | Move quickly and randomly |
| Compressibility | Almost none | Almost none | Easy to compress |
Notes · 2 of 5
Changes of state
Changing state is a physical change: the particles stay the same, only their arrangement and energy change.
If you heat a solid, its particles gain energy and vibrate faster. At the melting point the forces holding the regular arrangement together are overcome and the solid becomes a liquid. Heating further raises the temperature until the boiling point, where particles have enough energy to escape the liquid completely and form a gas.
Solid → liquid
Melting
Particles gain energy and break free of their fixed positions.
Liquid → gas
Boiling / evaporating
Particles gain enough energy to spread out and move freely.
Gas → liquid
Condensing
Particles lose energy and come back close together as a liquid.
Liquid → solid
Freezing
Particles lose energy and settle into a regular arrangement.
Evaporation happens at the surface of a liquid below its boiling point: the fastest particles escape. It can be sped up by warming the liquid, increasing its surface area, or blowing air across it. A pure substance melts and boils at fixed, sharp temperatures; an impure one melts over a range and at a lower temperature.
Notes · 3 of 5
Heating and cooling curves (Supplement)
A temperature–time graph shows where a change of state is happening.
If you heat a solid and record its temperature against time, the graph has flat sections. On a heating curve, the temperature rises while the solid warms, stays flat while it melts, rises again while the liquid warms, stays flat again while it boils, then rises once more. A cooling curve is the mirror image, with plateaus at the condensation and freezing points.
- The sloping sections show a single state being warmed or cooled.
- The flat sections show a change of state at a constant temperature.
- The longer the flat section, the more energy the change of state needs.
Notes · 4 of 5
Diffusion
Particles spread out by random motion until they are evenly mixed.
Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient. The smell of perfume spreading across a room, and a drop of ink spreading through water, are both examples. Diffusion happens fastest in gases, slower in liquids, and hardly at all in solids — because gas particles move quickly and are far apart.
Supplement: at the same temperature, a gas with a lower relative molecular mass (Mᵣ) diffuses faster than one with a higher Mᵣ. In a tube with both gases, ammonia (Mᵣ = 17) diffuses faster than hydrogen chloride (Mᵣ = 36.5), so the white ring of ammonium chloride forms nearer the hydrogen chloride end.
Notes · 5 of 5
Gas pressure and volume (Supplement)
Squeezing a gas raises its pressure; warming it increases its volume at constant pressure.
Gas particles collide with the walls of their container, creating pressure. Reducing the volume gives the particles less space and more collisions per second, so the pressure rises. For a fixed mass of gas at constant temperature, pressure and volume are inversely proportional:
- 1Write the relationship
p₁V₁ = p₂V₂ at constant temperature.
- 2Substitute
Put in the known pressure and volume pairs, keeping units consistent.
- 3Solve and check
Rearrange for the unknown. A smaller volume must give a larger pressure.
Revise
Key terms
The words examiners expect you to use precisely.
- Kinetic particle theory
- The model in which all matter is made of particles that are in constant motion; their arrangement and energy explain the states of matter.
- Melting point
- The temperature at which a solid changes into a liquid.
- Boiling point
- The temperature at which a liquid changes into a gas throughout the liquid.
- Freezing
- The change of state from liquid to solid as particles lose energy and form a regular arrangement.
- Condensing
- The change of state from gas to liquid as particles lose energy and come close together.
- Evaporation
- The loss of the fastest particles from the surface of a liquid at a temperature below its boiling point.
Show 3 more key termsShow fewer key terms
- Diffusion
- The spreading of particles from a region of higher concentration to one of lower concentration by random motion.
- Relative molecular mass (Mᵣ)
- The sum of the relative atomic masses of the atoms in a molecule; lighter molecules diffuse faster.
- Pressure
- The force per unit area caused by gas particles colliding with the walls of their container.
Revise
Common mistakes
Where students most often lose marks in this topic — and what to do instead.
Mistake: Saying particles "expand" when a gas is heated.
The particles themselves do not get bigger; they move faster and the spaces between them increase.
Instead: The particles stay the same size — the distance between them changes.
Mistake: Thinking the temperature rises while a solid melts.
Energy is being used to break the forces between particles, not to raise the temperature.
Instead: The temperature is constant during a change of state.
Show 8 more mistakesShow fewer mistakes
Mistake: Confusing boiling with evaporation.
Boiling happens throughout the liquid at a fixed temperature; evaporation happens at the surface at any temperature.
Instead: Boiling = throughout the liquid, at the boiling point. Evaporation = at the surface, below the boiling point.
Mistake: Saying that heavy particles diffuse slower "because they are bigger".
The key idea in the syllabus is relative molecular mass, not physical size.
Instead: A larger Mᵣ means a slower rate of diffusion at the same temperature.
Mistake: Forgetting to use matching units in p₁V₁ = p₂V₂.
Pressure and volume may be given in different units for the two states.
Instead: Convert to the same units before substituting.
Mistake: Claiming diffusion happens easily through solids.
Solid particles can only vibrate in fixed positions, so they cannot move past one another.
Instead: Diffusion is fastest in gases, slower in liquids, and very slow/negligible in solids.
Mistake: Reading the second flat section of a heating curve as the melting point.
A heating curve has two plateaus: the first is at the melting point and the second at the boiling point.
Instead: The lower plateau is melting; the higher plateau is boiling.
Mistake: Saying diffusion is faster in liquids than in gases because liquid particles are closer together.
Closeness is the obstacle, not the help: in a liquid the moving particles keep colliding with tightly packed neighbours, which slows their progress.
Instead: Diffusion is fastest in gases, where particles travel quickly between widely spaced collisions.
Mistake: Stating that evaporation can only happen at the boiling point.
Evaporation happens at the surface at any temperature; boiling is the separate process that needs the boiling point.
Instead: Boiling occurs throughout the liquid at a fixed temperature; evaporation occurs at the surface below the boiling point.
Mistake: Assuming all particles in a gas move at exactly the same speed.
The temperature fixes the average kinetic energy, but individual particles are always moving faster and slower than that average.
Instead: There is a spread of particle speeds at any temperature.
Check yourself
Quick quiz
18 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 arrangement in a gas
Which statement best describes the particles in a gas?
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Practise
Topical questions
10 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this topic, in three tiers, each with a model answer, mark scheme and examiner tip.
Practise topical questionsExam practice
Practice paper
A 30-mark paper set out like a Cambridge paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Topic 1 Practice Paper
30 marks · 40 min · 6 pages
10 multiple-choice questions and structured questions with answer lines. Topic 1: States of matter.
Mark scheme
PROTopic 1 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.
Which paper covers this topic?
Paper 1 / 2
Multiple choice (45 min, 40 marks)
Core and Extended versions
Paper 3 / 4
Theory, short and structured (1 h 15, 80 marks)
Core and Extended versions
Paper 5 / 6
Practical test or Alternative to Practical (40 marks)
Experimental skills
Want a full paper? Try the Paper 2-style multiple choice, Paper 4-style structured or Paper 6-style practical sets.
Questions
Frequently asked questions
What is the kinetic particle theory in one line?
All matter is made of particles that are always moving; the arrangement of the particles and how much they move explain the properties of solids, liquids and gases.
Why does the temperature not change while a substance melts?
The energy supplied is used to overcome the forces between particles so they can move out of their fixed positions. It does not increase the particles’ kinetic energy, so the temperature stays constant.
What is the difference between boiling and evaporation?
Boiling happens throughout the liquid at the boiling point, a fixed temperature. Evaporation happens only at the surface, and can happen at any temperature below the boiling point.
What is diffusion and what affects its rate?
Diffusion is the random spreading of particles from a region of higher concentration to a region of lower concentration. It is faster at higher temperatures and for gases with a lower relative molecular mass.
Do I need p₁V₁ = p₂V₂ for IGCSE Chemistry?
The formula itself is not printed in the 0620 syllabus. Extended (Supplement) candidates must explain, using kinetic particle theory, how temperature and pressure change the volume of a gas; p₁V₁ = p₂V₂ is a handy way to express the inverse pressure–volume relationship for a fixed mass of gas at constant temperature.
Up next
Topic 2: Atoms, elements and compounds