Kinetic Particle Model and States of Matter
All matter is made of particles in constant motion. This one model explains states of matter, changes of state, gas pressure, expansion and diffusion — so it is worth getting exactly right.
1. The three states
| Solid | Liquid | Gas | |
|---|---|---|---|
| Arrangement | regular, closely packed | close together, irregular | far apart, random |
| Motion | vibrate about fixed positions | slide past each other | move rapidly in all directions |
| Forces between particles | strong | moderate | very weak / negligible |
| Spacing | very close | close | far apart |
| Shape | fixed | takes the container’s shape | fills the container |
| Volume | fixed | fixed | not fixed |
| Compressible? | no | almost no | yes |
In a solid the particles still move — they VIBRATE about fixed positions. Saying solid particles are stationary is wrong; they have kinetic energy, just not enough to break free.
Forces between particles are never truly zero, even in a gas — they are negligible, which is not the same as absent. Stating the force between particles is zero was a recorded error; “very weak” or “negligible” is the safe wording.
2. Temperature and kinetic energy
Temperature is a measure of the AVERAGE KINETIC ENERGY of the particles.
Higher temperature → particles move faster → more kinetic energy.
Kinetic energy depends on TEMPERATURE, not on mass or on which state has the “heaviest” particles. A recorded error claimed solids have the highest kinetic energy “because they have the highest mass”. At the same temperature, it is the gas particles that move fastest, because they are free to travel rather than merely vibrate.
Absolute zero (−273 °C, or 0 K) is the temperature at which particle motion is at a minimum.
Converting: K = °C + 273
3. Changes of state
| Change | From → To | Energy |
|---|---|---|
| Melting | solid → liquid | absorbed |
| Freezing / solidifying | liquid → solid | released |
| Boiling / evaporating | liquid → gas | absorbed |
| Condensing | gas → liquid | released |
| Sublimation | solid → gas directly | absorbed |
During a change of state:
The temperature stays CONSTANT, even though energy is still being supplied.
The energy goes into breaking the forces between particles (increasing potential energy) rather than making them move faster — so the kinetic energy, and therefore the temperature, does not change.
Mass is conserved throughout — only the arrangement and spacing change.
4. Gas pressure
Gas pressure is caused by particles COLLIDING with the container walls. Each collision exerts a tiny force, and the total force over the wall area gives the pressure.
Pressure is FORCE PER UNIT AREA. Tutors flagged the exact wording — use “force per unit area”, not “force on an area”. A recorded error applied force ÷ area incorrectly, so keep the definition and the formula (p = F/A) together.
Explaining a change in pressure — always in terms of collisions:
Increasing the temperature (constant volume):
- Particles gain kinetic energy and move faster
- They collide with the walls more often and with greater force
- So the pressure increases
Decreasing the volume (constant temperature):
- The particles have less space
- They hit the walls more frequently (the same number of particles, smaller area, shorter distances between collisions)
- So the pressure increases
Every gas pressure explanation needs both parts: the frequency of collisions and, where temperature changes, the force of each collision. A temperature change affects both; a volume change affects only the frequency.
Boyle’s law
At constant temperature for a fixed mass of gas: p V = constant, so p₁V₁ = p₂V₂
Pressure and volume are inversely proportional — halve the volume and the pressure doubles.
Example: 200 cm³ of gas at 100 kPa is compressed to 50 cm³ at constant temperature.
- p₂ = p₁V₁/V₂ = (100 × 200) ÷ 50 = 400 kPa
p₁V₁ = p₂V₂ and pV = constant say exactly the same thing — the first is just the second applied at two moments. Both are correct, and either can be quoted.
The conditions matter: constant temperature and a fixed mass of gas. Quote them if the question asks for the law in full.
5. Brownian motion
Brownian motion is the random, jerky movement of small visible particles (such as smoke particles in air) caused by collisions with fast-moving, invisible molecules.
What it shows:
- Molecules are in constant random motion
- Molecules are very small but have enough momentum to move much larger particles
- It is direct evidence for the kinetic particle model
Lighter particles move more erratically, because a collision changes their velocity more.
6. Diffusion, expansion and evaporation
Diffusion — particles spread from high to low concentration because of their random motion. It is faster in gases (particles move faster and are further apart) and faster at higher temperatures.
Thermal expansion — heating makes particles vibrate/move more, so they occupy more space on average. Solids expand least, gases most.
Evaporation — the fastest particles escape from the surface of a liquid. Because the fastest leave, the average kinetic energy of those remaining falls, so the liquid cools.
Evaporation happens at any temperature and only at the surface; boiling happens at a fixed temperature throughout the liquid.
7. Mistakes that cost marks
Saying solid particles don’t move.
Saying forces between gas particles are zero.
Linking kinetic energy to mass rather than temperature.
Saying the temperature rises during a change of state.
Explaining pressure without mentioning collisions.
Giving only frequency (or only force) in a pressure explanation.
Loose wording for pressure instead of “force per unit area”.
Forgetting the conditions for Boyle’s law.
Saying evaporation happens throughout the liquid.
Frequently asked questions
How are particles arranged in the three states? Solid: regular, closely packed, vibrating. Liquid: close, irregular, sliding. Gas: far apart, random, fast.
What does temperature measure? The average kinetic energy of the particles.
What happens to temperature during a change of state? It stays constant — the energy breaks bonds instead of increasing kinetic energy.
What causes gas pressure? Particles colliding with the container walls.
Why does heating a gas increase its pressure? Particles move faster, colliding more often and with greater force.
Why does compressing a gas increase its pressure? The particles hit the walls more frequently in the smaller volume.
What is Boyle’s law? pV = constant (so p₁V₁ = p₂V₂) at constant temperature for a fixed mass of gas.
What is Brownian motion? Random jerky motion of visible particles caused by collisions with molecules — evidence for the kinetic model.
Why does evaporation cool a liquid? The fastest particles escape, lowering the average kinetic energy of those left.
What is absolute zero? −273 °C (0 K) — minimum particle motion.
Quick revision checklist
- I can describe arrangement, motion and forces in all three states
- I know solid particles vibrate
- I know inter-particle forces in a gas are negligible, not zero
- I know temperature measures average kinetic energy
- I can name all the changes of state
- I know temperature is constant during a change of state, and why
- I can explain gas pressure using collisions
- I use “force per unit area”
- I can explain pressure changes from temperature and from volume
- I know pV = constant and its conditions
- I can describe Brownian motion and what it shows
- I can explain diffusion, expansion and evaporation with the model
- I know why evaporation causes cooling
- I can convert °C to K
These notes cover the kinetic particle model and states of matter in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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