IGCSE Physics: Kinetic Particle Model of Matter Practice Questions
The kinetic particle model describes matter as particles in constant motion. The arrangement, separation and motion of those particles explain the properties of solids, liquids and gases, and their average kinetic energy determines the temperature.
Sub-topic 2.1 of Cambridge IGCSE Physics 0625 overlaps with Chemistry but is examined with a physics emphasis on Brownian motion and gas behaviour. Every answer needs the particles named explicitly. The questions below cover the three states, Brownian motion, evaporation and the pressure and volume relationship.
What you need to know for Kinetic Particle Model of Matter
- SolidParticles are close together in a fixed regular arrangement, vibrating about fixed positions. Fixed shape and volume, and not compressible.
- LiquidParticles are close together but randomly arranged and able to move past one another. Fixed volume but takes the shape of the container.
- GasParticles are far apart, randomly arranged, moving quickly in all directions. No fixed shape or volume, and easily compressed.
- Brownian motionThe random jerky motion of small visible particles, such as smoke particles in air, caused by collisions with much smaller, faster moving, invisible air molecules. It is evidence that molecules exist and are in constant random motion.
- Temperature and kinetic energyThe temperature of an object is related to the average kinetic energy of its particles. Raising the temperature increases that average kinetic energy.
- EvaporationThe faster moving particles escape from the surface of a liquid. The average kinetic energy of the remaining particles falls, so the liquid cools. Evaporation is faster at higher temperature, larger surface area and with more airflow.
IGCSE Physics Kinetic Particle Model of Matter questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Smoke particles viewed through a microscope in a smoke cell appear to move in a random, jerky way. Explain this observation and state what it provides evidence for.
Show the worked answer
- The air in the cell contains molecules that are far too small to see, moving quickly in random directions.
- These molecules collide with the much larger smoke particles from all sides.
- At any instant the collisions are uneven, so there is a resultant force on the smoke particle in a random direction, and it moves that way until the next uneven set of collisions changes it.
- This is Brownian motion. It provides evidence that matter is made of particles and that those particles are in constant random motion.
Explain, in terms of particles, why a liquid has a fixed volume but takes the shape of its container.
Show the worked answer
- In a liquid the particles are close together, almost touching, held by forces of attraction between them.
- Because they are already close together, they cannot spread further apart, so the volume is fixed and the liquid cannot be compressed appreciably.
- The particles are arranged randomly rather than in a fixed lattice, and they have enough energy to move past one another.
- The liquid can therefore flow and take up the shape of whatever container it is placed in.
A fixed mass of gas is compressed slowly to half its original volume at constant temperature. Explain, in terms of particles, what happens to its pressure.
Show the worked answer
- The number of gas particles and their average speed are unchanged, because the mass and temperature are constant.
- Halving the volume means the particles are confined in a smaller space, so each particle travels a shorter distance between collisions with the walls.
- The number of collisions with the walls per second therefore doubles.
- A greater number of collisions per second on each unit of wall area means the pressure doubles. Pressure and volume are inversely proportional at constant temperature, so pV is constant.
Explain why a person feels cold when water evaporates from their skin.
Show the worked answer
- Particles in a liquid have a range of speeds and therefore a range of kinetic energies.
- Only the fastest particles at the surface have enough energy to overcome the attractive forces and escape as vapour.
- When those fastest particles leave, the average kinetic energy of the particles remaining in the liquid falls, so the liquid's temperature falls.
- The cooler water then takes thermal energy from the skin by conduction, so the skin cools and the person feels cold.
Common mistakes in this topic
- Describing liquid particles as far apart.
- Saying particles expand when a substance is heated. The particles gain energy and move further apart or faster.
- Confusing Brownian motion with diffusion.
- Saying compressed gas particles speed up at constant temperature.
- Explaining evaporation as boiling. Evaporation happens at any temperature and only at the surface.
Exam tips
- Every answer in this topic should name the particles and describe their arrangement, separation and speed.
- For Brownian motion, always say the colliding molecules are invisible and much smaller than the particle being observed.
- When temperature is constant, particle speed is constant. When volume is constant, collision frequency changes only with speed.
- Use average kinetic energy rather than energy when discussing temperature.
- For evaporation, the phrase most energetic particles escape carries the mark.
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Kinetic Particle Model of Matter FAQs
What is Brownian motion?
Brownian motion is the random jerky movement of small visible particles, such as smoke particles suspended in air, caused by uneven collisions with much smaller and faster moving molecules. It provides direct evidence that matter is made of particles that are in constant random motion.
Why does compressing a gas increase its pressure?
With the same number of particles moving at the same average speed confined in a smaller volume, each particle travels a shorter distance between collisions with the walls. The number of collisions per second on each unit of wall area increases, so the pressure rises. Halving the volume doubles the pressure at constant temperature.
Why does evaporation cool a liquid?
Only the fastest moving particles at the surface have enough energy to escape. When they leave, the average kinetic energy of the particles remaining behind is lower, so the temperature of the liquid falls. That cooler liquid then draws thermal energy from anything in contact with it.
What is the difference between evaporation and boiling?
Evaporation happens only at the surface of a liquid and can occur at any temperature, as the fastest particles escape. Boiling happens at a fixed temperature, the boiling point, and occurs throughout the liquid with bubbles of vapour forming inside it.
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Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.