Kinetic Particle Model of Matter: Complete O Level Physics Cheatsheet
O Level Physics, Chapter 8 · Read time: ~8 minutes
These notes come from TGC Academy's O Level Physics tuition in Singapore.
Key Formulas
Solids, liquids and gases
Solid: particles are closely packed in a regular arrangement, held by strong attractive forces, and vibrate about fixed positions. Fixed shape and volume, cannot be compressed.
Liquid: particles are closely packed but randomly arranged, and slide past one another. Fixed volume, but takes the shape of its container. Very hard to compress.
Gas: particles are far apart, with negligible forces between them, and move randomly at high speed. No fixed shape or volume, easily compressed.
Trick: When asked to explain a property, link it to particles in three parts: arrangement (how close), motion (how they move) and forces (how strongly held). For example, gases are easily compressed because the particles are far apart with large spaces between them.
Brownian motion
Smoke particles in air, seen under a microscope, move in a random, jerky path. This is Brownian motion.
It happens because the smoke particles are bombarded unevenly by fast-moving, invisible air molecules. It is evidence that molecules in a liquid or gas are in constant random motion.
Trap: The smoke particles are not moving by themselves. Say they are being hit by air molecules that are too small to see. Writing that the smoke particles are 'vibrating' loses the mark.
Temperature and gas pressure
When a body's temperature rises, the average kinetic energy of all its particles increases, so they move or vibrate faster.
Gas particles collide with the walls of their container. Each collision exerts a small force. The total force per unit area is the gas pressure.
Heating a gas at constant volume: particles move faster, hit the walls more frequently and harder, so pressure increases.
Compressing a gas at constant temperature: the same particles are in a smaller space, so they hit the walls more frequently, and pressure increases.
Trap: When volume decreases at constant temperature, the particles do not move faster. The pressure rises only because collisions with the walls become more frequent.
Worked Example
A sealed metal can of air is heated from 25°C to 80°C. Its volume does not change.
(a) State what happens to the average kinetic energy of the air particles. [1]
It increases, because the temperature has increased.
(b) Explain, in terms of particles, why the pressure inside the can increases. [3]
The air particles move faster.
They collide with the walls of the can more frequently, and with greater force each time.
So the force per unit area on the walls increases, meaning the pressure increases.
(c) Explain why it is dangerous to heat a sealed aerosol can. [1]
The pressure keeps rising as the temperature rises, and the can may burst or explode.
Why this question is a good test of the topic: full marks in part (b) need both ideas, more frequent collisions and harder collisions. Students who mention only one usually lose a mark.
Frequently Asked Questions
How are particles arranged in a solid, liquid and gas?
Solid: closely packed, regular, vibrating about fixed positions. Liquid: closely packed, random, sliding past each other. Gas: far apart, random, moving at high speed.
What is Brownian motion evidence for?
That molecules in a liquid or gas are in constant, random motion.
What happens to particles when temperature rises?
Their average kinetic energy increases, so they move or vibrate faster.
What causes gas pressure?
Gas particles colliding with the walls of their container. The force of these collisions per unit area is the pressure.
Why are gases easy to compress?
Their particles are far apart with large spaces between them, so they can be pushed closer together.
Particle explanations still costing you marks?
Small group O Level Physics classes at TGC Academy's Bishan, Bukit Timah and Potong Pasir centres, helmed by Andrew Seah, MOE Award-Winning Teacher and Marshall Cavendish textbook author.