E. Earth and Space Systems · Heat in the Environment

The Particle Theory of Matter

E2.1 A1.2 A1.5About 45 minutes

Learning goals

I can state the main ideas of the particle theory of matter.
I can use the particle theory to describe solids, liquids and gases.
I can use the particle theory to explain everyday observations, like smells spreading through a room.
I can explain why scientists use models to picture things too small to see.

Words to know

matter
Anything that has mass and takes up space.

particle
A tiny piece of matter, far too small to see, even with a microscope you would have at home or at school.

particle theory of matter
A scientific explanation that describes what all matter is made of and how its particles behave.

theory
In science, an explanation that is supported by a lot of evidence from many tests.

model
A drawing, object or idea that helps us picture something we cannot see directly.

diffusion
The spreading of particles of one substance through another, caused by particle motion.

force of attraction
A pull that holds particles together.

Remember from Grade 5

In Grade 5 you learned that matter is made of tiny particles that are always moving. You used a particle model to picture solids (particles packed in place), liquids (particles close together but sliding past each other) and gases (particles far apart and zooming around). You also learned that adding thermal energy makes particles move faster.

In Grade 7 you will turn that model into the full particle theory of matter and use it to explain heat: what it is, how it moves, and why it matters for our whole planet.

A mystery in the kitchen

Someone is baking cookies in the kitchen. A minute later, you can smell them in your bedroom, even though you never left your room. Nobody carried the smell to you. How did it get there?

Scientists answer questions like this with the particle theory of matter. It is one of the most useful ideas in all of science.

A tray of freshly baked chocolate chip cookies on a kitchen counter with faint steam rising. — Fresh cookies give off particles that move and spread out through the air. That is how the smell reaches your room without anyone carrying it. AI-generated image

The particle theory of matter

All matter is made of tiny particles. They are much too small to see.
Particles are always moving. They have energy of motion.
There are spaces between particles. The spaces are small in solids and liquids and very large in gases.
Particles attract each other. These pulls (forces of attraction) hold solids and liquids together.
Heat makes particles move faster. When particles gain energy, they move faster. When they lose energy, they slow down.
Each pure substance has its own kind of particle. All the particles of pure water are the same, and they are different from the particles of sugar or of oxygen.

A theory in science is not a guess. It is an explanation that has been tested again and again and is supported by a lot of evidence.

A particle model of a solid, a liquid and a gas. In the solid, particles are packed in rows and can only vibrate in place. In the liquid, particles are close together but jumbled and can slide past each other. In the gas, particles are far apart and move freely in all directions.

	Solid	Liquid	Gas
How particles are arranged	packed tightly in a regular pattern	close together, but not in a pattern	far apart, spread out
How particles move	vibrate (jiggle) in place	slide past each other	move quickly and freely in all directions
Attraction between particles	strong, holds them in place	weaker, lets them slide	very weak, particles move apart
Shape	keeps its own shape	takes the shape of its container	fills its whole container
Volume	fixed	fixed	changes: spreads out or can be squeezed

Comparing solids, liquids and gases using the particle theory.

Using the theory to explain things

The particle theory explains many things you see every day:

Smells spread because gas particles are always moving. They bump around and spread through the air. This spreading is called diffusion.
You can squeeze the air in a syringe or a bike pump, but you cannot easily squeeze water. Gas particles have big spaces between them. Liquid particles are already close together.
A drop of food colouring spreads through still water without stirring, because water particles are always moving and bumping the colour particles around.
Sugar seems to disappear in water. The sugar particles spread out into the spaces between the water particles.

Diffusion of a smell. The smell particles start near the cookies, but because all particles keep moving, they spread into the spaces between the air particles until some reach the far end of the house.

Worked example

Let's explain with particle theory: the cookie smell

Use the particle theory to explain how the smell of baking cookies reaches your bedroom.

What do we observe? We can smell the cookies far from the kitchen.
Which ideas from the theory fit? Matter is made of particles (the smell is particles in the air). Particles are always moving. Gas particles have large spaces between them.
Put it together. Particles of the cookie smell escape into the air as a gas. They keep moving and bumping into air particles, so they spread into the spaces between air particles, all through the house.
Bonus idea: hot cookies release smell particles faster, because heat makes particles move faster.

The smell reaches you by diffusion: smell particles are always moving and spread out through the spaces between the air particles until some reach your nose.

Just for fun

If you could shrink to the size of a particle, a perfectly still glass of water would look like a school hallway at the bell: billions of particles bumping and jostling, and not one of them ever sits down.

Why use a model?

Nobody can see a single particle of water with their eyes. Scientists use models (drawings, objects or ideas) to picture what is happening. A good model explains what we observe and helps us make predictions. But every model has limits: real particles are not little balls with outlines, and they do not have colours.

Hands-on activity

Model the spaces between particles

You need: two measuring cups (or two identical clear glasses), about 1 cup of large dried beans, about 1 cup of uncooked rice, a large bowl, a clear glass of water, and food colouring.

Part 1: Spaces between particles (A1.2)

Measure exactly 1 cup of beans and exactly 1 cup of rice.
Predict: if you mix them, will you get 2 cups?
Pour both into the bowl and mix well with your hands. Shake the bowl gently so the rice settles.
Pour the mix back into the measuring cups and measure the total.

What to look for: you get less than 2 cups, because the small rice grains fill the spaces between the beans. In the model, the beans and rice stand for the particles of two different substances. (Real liquids show this too: mixing some liquids gives slightly less volume than you would expect.)

Part 2: Moving particles

Fill the glass with water and let it sit until it is completely still (about 1 minute).
Gently add one drop of food colouring. Do not stir.
Watch for 5 minutes and draw what you see at the start, after 1 minute and after 5 minutes.

What to look for: the colour slowly spreads through the water on its own. This is evidence that water particles are always moving.

Communicate (A1.5): label your drawings with words from this lesson: particles, moving, spaces, diffusion.

Safety: dried beans and rice are a choking hazard for very young children, so keep them out of reach. Food colouring can stain clothes and counters. Do not eat the beans or rice afterwards.

Your turn

What are the six main ideas of the particle theory of matter?

In which state are particles held in a regular pattern and only vibrate in place?

Why does a gas fill its whole container?

What is diffusion? Give an example.

Why is it easy to squeeze the air in a bike pump, but hard to squeeze water?

What is the difference between a theory in science and a guess?

Name one limit of drawing particles as little circles.

More practice: warm-up

True or false: particles in a solid do not move at all.

What happens to particles when they are heated?

In which state are the spaces between particles the largest?

What holds the particles of a solid together?

Are the particles of water the same as the particles of sugar?

More practice: on your own

Use the particle theory to explain why a liquid takes the shape of its container but keeps the same volume.

Food colouring spreads faster in hot water than in cold water. Which idea of the particle theory explains this?

When you stir sugar into a glass of water, the water level rises only a little. Explain using the particle theory.

Someone opens a jar of pickles at the other side of the room. Explain in 3 steps how you come to smell it.

In the beans-and-rice model, what do the beans and rice stand for? What does the result show?

Challenge

A balloon filled with air slowly gets smaller over a few days, even though it has no hole you can see. Use the particle theory to suggest why.

Explain why a solid has a fixed shape but a gas does not, using two ideas from the particle theory (movement and attraction).

Design a better model: suggest a way to show that particles are always moving that a drawing cannot show.

Check your understanding

Quiz

Which statement is part of the particle theory of matter?

In which state do particles slide past each other but stay close together?

The smell of a skunk spreads through the air. What is this spreading called?

Why can a gas be squeezed into a smaller space?

What happens to particles when they lose energy?

Why do scientists use models of particles?

For parents and teachers

Big idea (E2.1): the particle theory of matter is the foundation for this whole unit. Every later lesson (temperature, expansion, changes of state, conduction, convection) uses it. The activity practises investigating (A1.2) and communicating with labelled diagrams (A1.5).

How to help: ask your child to explain everyday things "using particles": why a puddle disappears, why you can smell supper cooking, why a sponge can be squeezed but a rock cannot. Encourage full sentences that name an idea from the theory.

Common difficulties:

Thinking particles in solids are still. They vibrate in place.
Thinking particles themselves expand or melt. Particles do not change size; their motion and spacing change.
Thinking a theory is just a guess. In science, a theory is well tested and supported by evidence.

Safety: dried beans and rice are a choking hazard for young children. Food colouring stains.