Year 9 · Science · Chemical sciences
The particle model of matter
Students use the idea that all matter is made of tiny moving particles to explain the properties of solids, liquids and gases, diffusion and expansion.
Awaiting educator review. This material was drafted by our content team with AI assistance and is waiting for review by a qualified educator. Please check it suits your child before using it. Created 10 October 2026.
These lessons were written around Australian Curriculum strands and matched to New Zealand year levels by age (NZ Year 1 is the first year of school). They are not yet mapped to The New Zealand Curriculum, so check them against your own learning programme.
Learning goals
Learners will
- Describe the particle model of matter.
- Use the particle model to explain the properties of solids, liquids and gases.
- Investigate how temperature affects the speed of diffusion.
Success looks like
- I can draw particle diagrams for a solid, a liquid and a gas.
- I can explain why gases can be compressed but solids cannot easily be.
- I can explain why food colouring spreads faster in hot water.
The big idea
The particle model says that all matter is made of tiny particles that are always moving, that attract one another, and that have empty space between them.
- In a solid, particles are packed tightly in fixed positions and vibrate on the spot. This is why solids keep their shape and are very hard to compress.
- In a liquid, particles are close together but can slide past each other. This is why liquids flow and take the shape of their container but keep their volume.
- In a gas, particles are far apart and move quickly in all directions. This is why gases fill their container and can be compressed.
When matter is heated, its particles gain kinetic energy and move faster. In a solid they vibrate more and push slightly further apart, which causes expansion. Bridges and railway lines have expansion gaps for hot Australian summers.
Diffusion happens because particles are always moving. When you add food colouring to water, its particles collide with water particles and gradually spread out. In hot water, particles move faster, so diffusion is quicker.
Models are useful but not perfect. The particle model does not show what the particles look like or why they attract each other, but it explains a great deal of what we observe.
Worked examples
Diffusion in hot and cold water
Does food colouring spread faster in hot or cold water?
- Fill one glass with cold water and one with hot tap water. Let them settle for a minute so the water is still.
- Gently add one drop of food colouring to each at the same time, without stirring.
- Time how long until each glass is evenly coloured.
- The hot glass becomes evenly coloured much sooner.
- Explanation: water particles in the hot glass have more kinetic energy, so they move faster and spread the dye more quickly.
Answer: Faster in hot water, because the particles move faster.
Compressing air
Why can you push the plunger of a sealed syringe full of air, but not one full of water?
- In a gas, particles are far apart with lots of empty space.
- Pushing the plunger moves gas particles closer together.
- In a liquid, particles are already touching, so there is almost no space to squeeze.
Answer: Gas particles have space between them; liquid particles do not.
Practice check
Have a go, then check your answers. Each answer comes with an explanation.
Answer guide for parents
Which best describes particles in a solid?
Close together, vibrating in fixed positions — Solid particles are tightly packed and vibrate on the spot.
Why can gases be compressed easily?
There are large spaces between their particles — Large gaps between gas particles allow them to be pushed closer together.
What happens to particles when a substance is heated?
They move faster — Heating gives particles more kinetic energy; the particles themselves do not grow.
Why do railway lines have small gaps between sections?
To allow for expansion in hot weather — Steel expands when hot; gaps stop the rails buckling.
You can smell a barbecue from next door because of…
Diffusion — Smell particles diffuse through the air from the barbecue to your nose.
What is the spreading of particles from high to low concentration called?
diffusion — This is diffusion.
Which state of matter has particles that are close together but can slide past each other?
liquid — Liquid particles touch but can move past one another.
What type of energy do particles gain when they move faster? (one word) ______ energy
kinetic — Energy of movement is kinetic energy.
Watch out for
- Students often think particles themselves expand when heated. The particles stay the same size; they move more and spread apart.
- Some think there is air between particles. Between particles there is empty space.
- Some think particles in a solid do not move. They vibrate constantly.
Tips for parents
- Ask your child to draw particle diagrams and explain each one aloud.
- Keep the glasses completely still during the diffusion test; any stirring spoils the result.
- Safety: use hot tap water only, not boiling water from a kettle; never use a syringe with a needle.
Go further
- Put a deflated balloon over the neck of an empty bottle and stand the bottle in hot tap water, then in iced water. Explain the result using particles.
- Spray a little perfume or air freshener in one corner of a room and time how long until people at different distances smell it.
- Research why the Sydney Harbour Bridge's arch rises and falls slightly with temperature.