60-minute lesson
Year 8 Science: The particle model of matter
Lesson objective
- 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 criteria:
- 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.
Materials
- Two identical clear glasses
- Hot water from the tap (not boiling) and cold water from the fridge
- Liquid food colouring
- A timer
- A clean syringe without a needle, or a plastic bottle with a lid
Introduction
6 minIntroduce today's words:
- particle: A tiny piece of matter, far too small to see, such as an atom or molecule.
- particle model: A scientific model stating that all matter is made of tiny particles that are always moving and attract one another.
- diffusion: The spreading of particles from where they are concentrated to where they are less concentrated.
- kinetic energy: The energy of movement.
- expansion: An increase in size, often caused by heating, because particles move more and spread apart.
- compress: To squeeze into a smaller volume.
Ask your child what they already know about the particle model of matter.
Explanation
12 minThe 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
9 minDiffusion in hot and cold water
Does food colouring spread faster in hot or cold water?
Step 1: Fill one glass with cold water and one with hot tap water. Let them settle for a minute so the water is still.
Step 2: Gently add one drop of food colouring to each at the same time, without stirring.
Step 3: Time how long until each glass is evenly coloured.
Step 4: The hot glass becomes evenly coloured much sooner.
Step 5: 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?
Step 1: In a gas, particles are far apart with lots of empty space.
Step 2: Pushing the plunger moves gas particles closer together.
Step 3: 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.
Guided practice (do together)
12 min1. Which best describes particles in a solid?
(a) Far apart and moving fast (b) Close together, vibrating in fixed positions (c) Close together and sliding past each other (d) Not moving at all
2. Why can gases be compressed easily?
(a) Their particles are very small (b) Their particles are hot (c) There are large spaces between their particles (d) Their particles do not move
3. What happens to particles when a substance is heated?
(a) They move faster (b) They get bigger (c) They stop moving (d) They melt
4. Why do railway lines have small gaps between sections?
(a) To save steel (b) To make a clicking noise (c) To let rain drain away (d) To allow for expansion in hot weather
Independent practice
15 min5. You can smell a barbecue from next door because of…
(a) Condensation (b) Diffusion (c) Freezing (d) Compression
6. What is the spreading of particles from high to low concentration called?
7. Which state of matter has particles that are close together but can slide past each other?
8. What type of energy do particles gain when they move faster? (one word) ______ energy
Questions to check understanding
- Can you draw particle diagrams for a solid, a liquid and a gas?
- Can you explain why gases can be compressed but solids cannot easily be?
- Can you explain why food colouring spreads faster in hot water?
- What was the trickiest part today?
Answer guide
1. Close together, vibrating in fixed positions — Solid particles are tightly packed and vibrate on the spot.
2. There are large spaces between their particles — Large gaps between gas particles allow them to be pushed closer together.
3. They move faster — Heating gives particles more kinetic energy; the particles themselves do not grow.
4. To allow for expansion in hot weather — Steel expands when hot; gaps stop the rails buckling.
5. Diffusion — Smell particles diffuse through the air from the barbecue to your nose.
6. diffusion — This is diffusion.
7. liquid — Liquid particles touch but can move past one another.
8. kinetic — Energy of movement is kinetic energy.
Review
6 minRecap the success criteria together. Watch for these common misconceptions:
- 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.
Extension activities
- 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.
Suggested follow-up
- Revisit the particle model of matter tomorrow with two or three quick questions from memory.
- Try the online practice check for this topic and look at any questions that need another go.
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.