Brownian Motion, Gas Pressure and the Gas Laws
Gases are the clearest demonstration of the particle model. Brownian motion is the evidence for it, and gas pressure is the model’s most useful prediction.
1. Brownian motion
Brownian motion is the random, jerky movement of small visible particles suspended in a fluid — such as smoke particles in air, viewed through a microscope.
The cause:
The visible particles are being bombarded unevenly by fast-moving, invisible air molecules. At any instant, more molecules strike one side than another, giving the particle a random push.
What it proves:
- Molecules are in constant, random motion
- Molecules are very small but move fast enough to have appreciable momentum
- It is direct evidence for the kinetic particle model
Explain it in terms of the SMALLER, invisible molecules hitting the LARGER, visible particles. Confusion over which particles are which was recorded — you see the smoke particles, and it is the air molecules you cannot see that are doing the hitting.
Dust and smoke particles stay suspended because of constant collisions with air molecules — not because they are weightless. A recorded error attributed it to their weight. Gravity acts on them, but the continual random bombardment keeps them aloft.
Lighter particles move more erratically, because each collision changes their velocity more.
2. Gas pressure from the particle model
Gas pressure is caused by particles COLLIDING with the walls of the container.
Each collision exerts a small force on the wall; the total force spread over the wall’s area gives the pressure.
The two things that determine the pressure:
How OFTEN the particles hit the walls (the frequency of collisions) How HARD they hit (the force of each collision)
Every explanation on this page comes back to those two factors.
3. Pressure and volume — Boyle’s law
At constant temperature, for a fixed mass of gas: p V = constant, so p₁ V₁ = p₂ V₂
Pressure and volume are INVERSELY proportional.
The particle explanation:
Reducing the volume gives the particles less space, so they travel shorter distances between collisions and hit the walls more frequently. The speed of the particles is unchanged (the temperature is constant), so each collision is just as hard — but there are more of them, so the pressure rises.
Example: 400 cm³ of gas at 150 kPa is compressed to 100 cm³ at constant temperature.
- p₂ = (150 × 400) ÷ 100 = 600 kPa
Increasing the pressure DECREASES the volume — they move in opposite directions. This was recorded wrong in both directions in lessons. Sanity-check: squeezing a gas into a smaller space must make it harder to squeeze, so the pressure is higher.
State the conditions: constant temperature, fixed mass of gas. They are part of the law.
On a graph: p against V is a curve (a hyperbola); p against 1/V is a straight line through the origin, which is the usual way to demonstrate the law.
4. Pressure and temperature (constant volume)
At constant volume, increasing the temperature increases the pressure.
The particle explanation:
Higher temperature → particles have more kinetic energy and move faster → they hit the walls more often and with greater force → the pressure rises.
Note that BOTH factors change here, unlike a volume change where only the frequency changes. Give both for full marks.
State the condition — “at constant volume”. A recorded error gave “as temperature increases, pressure increases” with no condition. In a balloon, which can expand, the volume is not constant, so the outcome is different.
Absolute zero is the temperature at which particle motion is minimal, so the pressure would be zero: −273 °C = 0 K.
Convert to kelvin for gas temperature relationships: K = °C + 273.
5. Volume and temperature (constant pressure)
At constant pressure, increasing the temperature increases the volume.
The particles move faster and push the walls further apart until the pressure balances again — which is why a balloon expands when warmed.
6. Answering gas questions
The structure that earns full marks:
- State what happens to the particles’ speed / kinetic energy (if the temperature changed)
- State what happens to the frequency of collisions with the walls
- State what happens to the force of each collision (if relevant)
- Conclude with the effect on pressure
Never just state the outcome. “The pressure increases” is the conclusion, not the explanation — the marks are in the collision reasoning.
7. Mistakes that cost marks
Reversing the pressure–volume relationship.
Omitting the conditions (constant temperature, constant volume, fixed mass).
Explaining pressure without mentioning collisions.
Giving only frequency when both frequency and force change.
Saying the particles themselves expand or shrink when the gas is compressed — the spacing changes, not the particles.
Confusing which particles are visible in Brownian motion.
Attributing suspended dust to its weight rather than collisions.
Not converting to kelvin where required.
Frequently asked questions
What is Brownian motion? The random, jerky motion of visible particles caused by collisions with invisible fast-moving molecules.
What does it show? That molecules are in constant random motion and are small but fast-moving.
Why don’t smoke particles fall to the floor? Because of continual random collisions with air molecules.
What causes gas pressure? Particles colliding with the container walls.
What is Boyle’s law? pV = constant at constant temperature for a fixed mass of gas.
What happens to volume if pressure increases? It decreases — they are inversely proportional.
Why does compressing a gas raise its pressure? The particles hit the walls more frequently in the smaller space.
Why does heating a gas at constant volume raise the pressure? Particles move faster, hitting the walls more often and harder.
What is absolute zero? −273 °C (0 K) — minimum particle motion.
Do gas particles get bigger when heated? No — they move faster and spread further apart.
Quick revision checklist
- I can describe Brownian motion and what causes it
- I know which particles are visible and which do the hitting
- I know what Brownian motion proves
- I can explain gas pressure using collisions
- I know pressure depends on collision frequency and force
- I know pV = constant and its conditions
- I can explain the pressure–volume relationship with particles
- I know pressure and volume are inversely proportional
- I can explain the pressure–temperature relationship at constant volume
- I state the condition in every gas law
- I can convert °C to K
- I structure explanations as speed → frequency → force → pressure
These notes cover Brownian motion, gas pressure and the gas laws 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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