IGCSE Physics Particle Model Questions and Worked Solutions (2.1.2) | 0625

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IGCSE Physics 0625 · Topic 2.1.2

IGCSE Physics Particle Model Questions and Worked Solutions

50 topic-wise practice questions on Particle Model (Unit 2: Thermal Physics), written to the Cambridge IGCSE Physics 0625 syllabus for 2026–2028, each with a step-by-step worked solution.

50 questions24 Core16 Extended10 Challenge124 marksTextbook §2.1 · pp. 86–99
Questions tagged Core are for all candidates (Papers 1 & 3). Questions tagged Extended are Supplement content (Papers 2 & 4). Questions 41–50 are harder Challenge questions. Try each question first, then tap Show solution to check your working.

Topic guide: Particle Model

The kinetic particle model explains the properties of solids, liquids and gases in terms of the arrangement, separation and motion of their particles. Brownian motion provides evidence that particles are constantly moving.

Key formulas

No equations to learn in this subtopic — temperature links to average kinetic energy of particles.

Prerequisite knowledge

  • States of matter (2.1.1)
  • Kinetic energy (1.7.1)

Exam tips & common mistakes

  • Describe all three: arrangement, separation AND motion
  • Say 'molecules' (not 'particles') for what hits the smoke specks in Brownian motion
  • Link pressure changes to frequency AND force of collisions

Key facts from the syllabus (2.1.2)

  • Solids: particles close together in a regular pattern, vibrating about fixed positions
  • Liquids: particles close together, irregular arrangement, moving past each other
  • Gases: particles far apart, random arrangement, moving quickly in all directions
  • Higher temperature means higher average kinetic energy of particles; absolute zero (−273 °C) is the lowest possible temperature
  • Gas pressure is caused by particles colliding with the walls of the container
  • Brownian motion: microscopic particles (smoke, pollen) move randomly because they are hit by fast, invisible molecules
  • Extended forces between particles are strongest in solids and weakest in gases; pressure = force of collisions ÷ area

Textbook: §2.1 Kinetic particle model of matter, pp. 86–99

1
Core[3 marks]

Describe the arrangement, separation and motion of particles in a solid.

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Method: Regular lattice, close together, vibrate.

Regular pattern, very close together, vibrate about fixed positions
2
Core[3 marks]

Describe the arrangement, separation and motion of particles in a liquid.

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Method: Random, close, slide past.

Irregular, close together, move around and slide past each other
3
Core[3 marks]

Describe the arrangement, separation and motion of particles in a gas.

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Method: Random, far apart, fast.

Random, far apart, move quickly in all directions
4
Core[2 marks]

State the relationship between the temperature of a gas and the motion of its particles.

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Method: Higher T → faster particles.

The higher the temperature, the greater the average kinetic energy (speed) of the particles
5
Core[2 marks]

State what is meant by absolute zero and give its value in °C.

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Method: Lowest possible temperature.

The lowest possible temperature, −273 °C, where particles have the least kinetic energy
6
Core[3 marks]

Explain how a gas exerts a pressure on the walls of its container.

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Method: Particle collisions with walls.

Particles collide with the walls and exert forces on them; force per unit area is the pressure
7
Core[2 marks]

Pollen grains in water are seen to move in a random, jerky way. Name this motion.

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Method: Brownian motion.

Brownian motion
8
Core[3 marks]

Explain the random motion of smoke particles in air seen through a microscope.

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Method: Collisions with air molecules.

Smoke particles are hit unevenly by fast-moving, invisible air molecules from random directions
9
Core[1 mark]

State what Brownian motion is evidence for.

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Method: Kinetic particle model.

The kinetic particle model of matter (particles are in constant random motion)
10
Core[2 marks]

In a smoke cell, the bright specks seen are not air molecules. Explain why air molecules cannot be seen.

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Method: Too small.

Air molecules are far too small to be seen with a microscope; only the larger smoke particles scatter light
11
Core[2 marks]

Explain why gases are easy to compress in terms of particles.

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Method: Large spaces between particles.

Particles are far apart with large spaces between them
12
Core[2 marks]

A gas is heated in a sealed container. Explain why the pressure increases.

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Method: Faster particles collide harder and more often.

Particles move faster, so they hit the walls more often and with greater force
13
Core[2 marks]

Compare the spacing of particles in a liquid and a gas.

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Method: Close vs far.

Liquid particles are close together; gas particles are far apart
14
Core[2 marks]

Explain why solids are difficult to compress.

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Method: Particles touching.

Particles are already very close together
15
Core[2 marks]

State what happens to the motion of particles in a solid when it is heated.

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Method: Vibrate more.

They vibrate faster/with larger amplitude
16
Core[2 marks]

State what happens to the particles of a gas at absolute zero.

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Method: Minimum KE.

They have the least possible kinetic energy
17
Core[2 marks]

Explain why a gas spreads to fill a room.

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Method: Random motion.

Gas particles move randomly at high speeds in all directions
18
Core[2 marks]

Explain why a gas has a lower density than its liquid.

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Method: Spacing.

The particles are much further apart, so the same mass takes up more volume
19
Core[2 marks]

Describe the arrangement of particles when a liquid freezes.

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Method: Regular lattice.

They become fixed in a regular arrangement, vibrating about fixed positions
20
Core[2 marks]

A sealed syringe of air is heated. Describe what happens to the speed of the air particles.

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Method: Faster.

They move faster on average
21
Core[3 marks]

Describe what is seen in a smoke cell viewed through a microscope.

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Method: Bright specks, random jerky motion.

Tiny bright specks of smoke moving about in a random, jerky, zigzag way
22
Core[2 marks]

State why the smoke particles in a smoke cell keep moving and do not settle.

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Method: Continuous collisions.

Air molecules keep colliding with them randomly
23
Core[2 marks]

Explain why pressure is produced when gas particles hit a wall.

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Method: Force.

Each collision exerts a force on the wall; the many forces spread over the area produce pressure
24
Core[2 marks]

State what happens to the pressure in a sealed container if more gas is pumped in at the same temperature.

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Method: More collisions.

The pressure increases because there are more frequent collisions with the walls
25
Extended[2 marks]

Explain why solids have a fixed shape in terms of forces between particles.

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Method: Strong forces hold particles in fixed positions.

Strong attractive forces hold particles in fixed positions
26
Extended[2 marks]

Explain why liquids can flow but solids cannot.

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Method: Weaker forces in liquids allow particles to move past each other.

In liquids the forces are weaker, so particles can slide past each other
27
Extended[2 marks]

Explain why gases have no fixed volume in terms of forces between particles.

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Method: Negligible forces.

Forces between gas particles are very small, so particles move apart freely
28
Extended[3 marks]

Explain pressure changes of a gas in terms of force per unit area.

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Method: Momentum change at walls gives force.

Each collision exerts a force on the wall; the total force divided by area is the pressure; more frequent or harder collisions increase pressure
29
Extended[2 marks]

In Brownian motion, explain why the microscopic particles move.

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Method: Light, fast molecules hit heavier particles.

Collisions with light, fast-moving molecules push the much larger particles in random directions
30
Extended[1 mark]

State the correct term for the tiny, fast-moving particles in the air that hit smoke particles.

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Method: Not 'particles' loosely.

Molecules (air molecules)
31
Extended[2 marks]

Explain why the movement of the smoke particles is jerky.

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Method: Uneven numbers of collisions on each side.

At any instant more molecules hit one side than the other, giving a resultant force that changes direction randomly
32
Extended[2 marks]

Explain why larger particles in a suspension show less Brownian motion than smaller ones.

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Method: More collisions average out; larger mass.

Larger particles have more mass and receive more balanced collisions, so their motion is smaller
33
Extended[2 marks]

Explain why the particles of a gas move freely.

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Method: Weak forces.

The forces between them are very weak because they are far apart
34
Extended[2 marks]

Compare the forces between particles in solids, liquids and gases.

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Method: Strongest in solid.

Strongest in solids, weaker in liquids, very weak in gases
35
Extended[3 marks]

Explain, in terms of momentum change, why a faster gas particle exerts a larger force on a wall.

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Method: Δp larger.

A faster particle has a larger change in momentum when it rebounds, and force = rate of change of momentum
36
Extended[2 marks]

Explain why the pressure of a gas is the same on all walls of its container.

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Method: Random motion in all directions.

Particles move randomly in all directions, so collisions are spread evenly over all walls
37
Extended[2 marks]

Pollen grains in water move less when the water is cooled. Explain why.

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Method: Slower molecules.

Water molecules move more slowly, so collisions with the pollen are less forceful
38
Extended[2 marks]

Explain why Brownian motion shows that air molecules are moving even though they cannot be seen.

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Method: Inference.

The visible particles are pushed randomly, which can only be caused by collisions with moving, invisible molecules
39
Extended[2 marks]

State the relationship between the average kinetic energy of particles and the temperature in kelvin.

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Method: Proportional.

Average kinetic energy is proportional to the temperature in kelvin
40
Extended[2 marks]

Explain why a liquid has a fixed volume even though its particles can move.

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Method: Attractive forces.

The attractive forces are strong enough to keep the particles close together

Challenge questions

Harder, multi-step questions at the top end of the exam. Attempt these once you are confident with the questions above.

41
ChallengeCore[4 marks]

Use the particle model to explain why a solid melts when heated.

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Method: Vibrations overcome positions.

Particles gain energy and vibrate more strongly until they break free of their fixed positions and can move past each other
42
ChallengeCore[4 marks]

Explain, using particles, why a sealed can of gas exerts more pressure when heated.

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Method: Speed and collisions.

Particles move faster, hitting the walls more often and with more force, so force per unit area increases
43
ChallengeCore[4 marks]

Describe and explain the motion of smoke particles in a smoke cell, and explain why it is evidence for moving air molecules.

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Method: Brownian motion.

They move in a random, jerky way because they are hit unevenly by invisible, fast-moving air molecules; the molecules must be moving to cause this
44
ChallengeCore[3 marks]

Explain why absolute zero is the lowest possible temperature in terms of particle motion.

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Method: Minimum KE.

At absolute zero particles have the minimum possible kinetic energy, so they cannot be slowed any further
45
ChallengeExtended[4 marks]

Explain why the pressure of a gas is greater if the particles have more mass but the same speed.

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Method: Momentum change.

Heavier particles have more momentum, so each collision gives a larger change in momentum and a larger force
46
ChallengeExtended[4 marks]

Explain why the smoke particles in Brownian motion move much more slowly than the air molecules that hit them.

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Method: Mass difference.

Smoke particles are much more massive, so collisions with tiny light molecules change their velocity only slightly
47
ChallengeExtended[4 marks]

Explain, in terms of forces between particles and their energy, why a liquid evaporates at room temperature.

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Method: Energetic particles escape attractions.

Some surface particles have enough kinetic energy to overcome the attractive forces of their neighbours and escape
48
ChallengeExtended[4 marks]

A gas is heated at constant volume from 300 K to 600 K. Explain, using particles, why its pressure doubles.

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Method: KE ∝ T.

The average kinetic energy doubles, so particles hit the walls harder and more often, doubling the pressure
49
ChallengeExtended[3 marks]

Explain why the particles in a solid are not still, even though the solid has a fixed shape.

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Method: Vibration.

They vibrate about fixed positions; strong forces stop them moving away
50
ChallengeExtended[4 marks]

Compare the forces between particles and their motion in solids, liquids and gases, and use this to explain why gases fill containers.

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Method: Weak forces in gas.

Strong forces in solids (vibrate only); weaker in liquids (slide past); very weak in gases (move freely in all directions), so gases spread to fill any container
All IGCSE Physics topics← 2.1.1 States of Matter2.1.3 Gases and the Absolute Scale of Temperature →

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