Kinetic particle model of matter
States of matter, particle arrangement and motion, changes of state, gas pressure and the gas laws.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 2.1.1Describe the arrangement, separation and motion of particles in solids, liquids and gases.
- 2.1.2Explain changes of state and evaporation in terms of particle behaviour.
- 2.1.3Explain gas pressure in terms of collisions of particles with the container walls.
- 2.1.4Recall and use the relationship between pressure and volume for a fixed mass of gas at constant temperature (pV = constant).Supplement
7 minute read
Particles, states and gas pressure
The three states
| State | Arrangement | Separation | Motion |
|---|---|---|---|
| Solid | regular, tightly packed | very close | vibrate about fixed positions |
| Liquid | irregular, close together | close | move around each other |
| Gas | random, far apart | far | fast, random, in all directions |
That table answers a surprising number of exam questions on its own.
Changes of state
Heating gives particles more kinetic energy. At the melting or boiling point, the extra energy is used to break the forces between particles rather than to raise the temperature — which is why a heating curve has flat sections.
Evaporation happens at any temperature, only from the surface, when the fastest particles escape. Because the fastest particles leave, the average kinetic energy of those remaining falls — so the liquid cools. This is why sweating works.
Gas pressure
Gas particles collide with the container walls. Each collision exerts a small force; the total force over the wall's area is the pressure.
- Raise the temperature at constant volume: particles move faster, collide more often and harder, so pressure rises.
- Reduce the volume at constant temperature: the same number of particles hit a smaller area more often, so pressure rises. For a fixed mass at constant temperature,
p₁V₁ = p₂V₂.
Think of it like this
Gas pressure is like hail on a roof. Harder hailstones (higher temperature) or a smaller roof (smaller volume) both mean more force on every square metre.
Worked examples
Method, step by step
A gas occupies 250 cm³ at a pressure of 100 kPa. The volume is reduced to 100 cm³ at constant temperature. Calculate the new pressure.
- 1p₁V₁ = p₂V₂
- 2100 × 250 = p₂ × 100
- 3p₂ = 25000 / 100
p₂ = 250 kPa
Common misconceptions
- Saying particles "expand" when a substance is heated. The particles stay the same size — they move further apart and vibrate more.
- Claiming evaporation only happens at the boiling point. It happens at all temperatures, from the surface only.
- Explaining pressure by "particles pushing each other" rather than by collisions with the walls.
In the exam
- Gas-pressure explanations need three steps: what happens to particle speed, what happens to the rate/force of collisions, and therefore what happens to pressure.
- For pV = constant questions, check that the temperature is stated as constant — otherwise the relationship does not apply.
- Never write that temperature "is" heat. Temperature is a measure of the average kinetic energy of the particles.