Particle Model of Matter

The particle model

The particle model of matter is a scientific theory that explains the behaviour of all matter — solids, liquids, and gases — in terms of tiny particles (atoms and molecules). The key ideas are:

All matter is made of particles too small to see
The spaces between particles are empty (not air — nothing)
Particles are always moving (even in solids they vibrate)
Particles in different states are arranged and behave differently

The three states of matter

SOLID

Closely packed, regular arrangement
Vibrate — do not move around
Strong forces between particles
Very small spaces
Fixed shape and volume

LIQUID

Loosely arranged, still close
Move and slide past each other
Weaker forces
Small spaces
Fixed volume, no fixed shape

GAS

No regular arrangement
Move very fast in all directions
Extremely weak forces
Very large spaces
No fixed shape or volume

Change of state

Heating gives particles more energy, causing them to move faster and further apart. Cooling removes energy, slowing particles down:

Process	Change	What happens to particles
Melting	Solid → Liquid	Gain energy; break free of fixed positions; slide past each other
Evaporation / Boiling	Liquid → Gas	Gain enough energy to escape; move very far apart
Condensation	Gas → Liquid	Lose energy; slow down; forces pull them closer
Freezing / Solidifying	Liquid → Solid	Lose more energy; settle into fixed positions

Diffusion

Diffusion is the net movement of particles from a region of high concentration to one of low concentration. It happens because particles are constantly moving randomly.

Diffusion is faster in gases than in liquids — gas particles move much faster and have more space
Example: a perfume bottle opened across the room — you smell it without any wind
Example: a drop of food colouring spreading through water (takes minutes to hours)

Curriculum link: Diffusion in gases and liquids also explains gaseous exchange in the lungs (Grade 9 Life & Living — oxygen diffuses from alveoli into blood, CO₂ diffuses out).

Density

Density describes how much mass is packed into a given volume. A material has high density if its particles are heavy and closely packed.

Solids are generally denser than liquids, which are denser than gases
Exception: Ice (solid water) is less dense than liquid water — that's why ice floats
Oil is less dense than water, so oil floats on water

Expansion and contraction

When materials are heated, particles move faster and push further apart — the material expands. When cooled, particles slow down and move closer — the material contracts.

During expansion and contraction, the number and size of particles stays the same — only the spaces between them change.

Pressure in gases

Gas particles are constantly moving and colliding with the walls of their container. These collisions create pressure. If you pump more gas into a container, there are more particles, more collisions, and therefore higher pressure. This explains why a tyre gets harder as you pump it up.

Write your answers in the spaces below. Use full sentences.

1. Describe the arrangement, movement and spacing of particles in a solid, liquid, and gas. Use the particle model of matter.

2. Explain what happens at the particle level when a solid melts and then when the liquid evaporates. What happens to the energy of the particles?

3. Explain diffusion and give one example in a gas and one example in a liquid. Why is diffusion faster in gases than in liquids?

4. A loaf of bread and a brick of the same size do not have the same mass. Explain this using the particle model of matter, referring to the concept of density.

5. Why does a metal bridge have small gaps built into it at regular intervals? Explain using the concept of expansion and contraction.

6. Explain why pumping more air into a bicycle tyre increases the pressure inside it. Use the particle model in your answer.

7. The table shows the mass and volume of four different blocks of material.

Block	Mass (g)	Volume (cm³)
A — Cork	24	100
B — Wood	70	100
C — Iron	790	100
D — Aluminium	270	100

(a) Calculate the density (mass ÷ volume) of each block, in g/cm³.
(b) Rank the four materials from LEAST dense to MOST dense.
(c) Water has a density of 1 g/cm³. Using your answers to (a), which of these materials would FLOAT if placed in water? Explain your reasoning.
(d) Using the particle model of matter, explain why the iron block has a much higher density than the cork block, even though both blocks have exactly the same volume (100 cm³).