C. Matter and Energy · Pure Substances and Mixtures

The Particle Theory of Matter

C2.1 A1.2 A1.4 A1.5About 45 minutes

Learning goals

I can state the main ideas (postulates) of the particle theory of matter.
I can use the particle theory to explain everyday things, like smells spreading and food colouring mixing.
I can explain why a scientific theory is a model that is supported by evidence.
I can carry out a fair test and describe my results with science vocabulary.

Words to know

matter
Anything that has mass and takes up space.

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

particle theory of matter
A scientific model that explains how matter behaves, using the idea that all matter is made of tiny moving particles.

postulate
One of the main statements that a theory is built on.

diffusion
The spreading out of particles of one substance through another, from where there are many to where there are few.

model
A simpler way to picture or explain something that is too small, too big or too complicated to see directly.

thermal energy
The energy of moving particles. Heating gives particles more of it.

Remember from Grade 5

In Grade 5 you learned that matter is anything with mass that takes up space. You used a simple particle model to describe solids (particles packed tightly), liquids (close but sliding past each other) and gases (far apart, moving fast). In Grade 6 you found out that everything is made of tiny atoms.

This year you will turn that model into a full scientific theory, and use it to explain mixtures, solutions and much more.

A mystery in the kitchen

Someone is making toast in the kitchen. A minute later, you can smell it in your bedroom. Nobody carried the smell to you. How did it get there?

Scientists explain puzzles like this with the particle theory of matter. A theory is not a guess. It is an explanation that has been tested many, many times, and it fits the evidence. Scientists keep it as long as it keeps working.

The particle theory: six main ideas

The main statements of a theory are called postulates. Here are the postulates of the particle theory, in plain words.

All matter is made of tiny particles. They are much too small to see.
Each pure substance has its own kind of particle. All the particles of one pure substance are the same. Different substances have different particles. (Water particles are all alike, and they are different from sugar particles.)
There are spaces between particles. In a gas, the spaces are very large compared with the particles.
Particles are always moving. Even in a solid, they vibrate (wiggle) in place.
Particles move faster when they are heated. Adding thermal energy speeds them up. Cooling slows them down.
Particles attract each other. The pull is strongest in solids and weakest in gases.

Water particles in cold water (left) and hot water (right). The same 12 particles move faster and spread out a little more when heated. The short lines show motion: longer lines mean faster motion. (Real particles are far too small to see.)

Using the theory to explain things

A good theory explains what we see. Here is how the particle theory explains the toast mystery:

The toast gives off particles of smell into the air (postulate 1).
Those particles are always moving (postulate 4), and there are big spaces between air particles (postulate 3).
So the smell particles bump and wander through the spaces between the air particles, slowly spreading through the house.

This spreading is called diffusion. Particles spread from where there are many of them to where there are few, until they are evenly mixed.

What we see	Postulates that explain it	Explanation
Food colouring spreads through still water	3, 4	Moving colouring particles slip into the spaces between water particles.
Food colouring spreads faster in hot water	4, 5	Hot water particles move faster, so they bump the colouring around faster.
Railway tracks and bridges have small gaps for summer heat	5	Heated particles move faster and push a little farther apart, so the metal expands.
A solid holds its shape	6	Strong attraction holds the particles in place; they can only vibrate.
You can squeeze the air in a syringe, but hardly squeeze water	3	Gas particles have big spaces between them; liquid particles are already close together.

Matching observations to the postulates that explain them.

Postulate 3 in action. Pushing the plunger squeezes the air into less space because gas particles have big spaces between them. The water's particles are already close together, so the plunger barely moves.

Worked example

Let's explain an observation with the particle theory

On a hot July day, a bicycle tire that was pumped up in the cool morning feels much harder by the afternoon. Use the particle theory to explain why.

Step 1. Say what you observe. The tire is harder in the afternoon.
Step 2. Name the change. The air inside got warmer in the sun.
Step 3. Pick the postulates. Particles are always moving (4), and they move faster when heated (5).
Step 4. Link them. The faster air particles hit the inside of the tire harder and more often. That pushes out on the tire more.
Step 5. Write a clear sentence. Use because to connect the observation and the theory.

The tire feels harder because heat from the sun made the air particles inside move faster, so they hit the inside walls of the tire harder and more often.

Just for fun

Why are particles terrible at freeze tag?

Even when they're "frozen" in a solid, they never stop wiggling.

A model, not a picture

Particles are not really little coloured balls. We draw them as circles to help us think. That is why the particle theory is called a model. Scientists can't see a single particle with an ordinary microscope, but the model correctly predicts what happens, so they keep using it.

Hands-on activity

Fair test: does temperature change how fast colouring spreads?

Ask a grown-up to help you with the hot water. Use hot water from the tap only, never boiling water from a kettle, and keep the cups on a table where they can't be knocked over.

You need: 2 identical clear glasses or jars, cold water from the fridge, hot tap water, liquid food colouring, a measuring cup, a timer and paper.

Question: Does food colouring spread faster in hot water or cold water?
Predict: Write your prediction and a because using the particle theory.
Plan a fair test. Change only one thing (the water temperature). Keep everything else the same: same glasses, same amount of water (for example 250 mL), same number of drops (2 drops), dropped from the same height. Let the water sit still for 1 minute so it stops swirling.
Add 2 drops to each glass at the same time. Do not stir or bump the glasses.
Observe after 30 seconds, 1 minute, 2 minutes and 5 minutes. Record what you see in a table, or take photos.
Conclude: Which spread faster? Was your prediction supported?
Share it: write two sentences that use the words particles, diffusion and thermal energy.

What to look for: the colouring spreads and mixes much faster in the hot water. The water particles there have more thermal energy, move faster and bump the colouring particles around faster. In the cold water, the colour spreads slowly and may stay in streaks for minutes.

Safety: hot tap water can scald. A grown-up fills the hot glass. Food colouring stains clothes and counters, so put down newspaper or a tray. Pour the water down the sink when you're done.

Your turn

What does the first postulate of the particle theory say?

What happens to particles when they are heated?

Are particles in a solid moving? Explain.

What is diffusion? Give an example.

Why is it easy to squeeze the air in a closed syringe?

Are the particles of water the same as the particles of sugar? Which postulate tells you?

Why do scientists call the particle theory a model?

More practice: warm-up

True or false: particles in a gas are far apart.

True or false: particles stop moving when something is cold.

Which postulate says particles pull on each other?

What do we call the energy of moving particles?

Can you see a single particle of water with a magnifying glass?

More practice: on your own

A balloon is left in a cold car in January. It shrinks a little. Explain with the particle theory.

Why can you smell cookies baking from another room, but not right away?

Why do concrete sidewalks have cracks or gaps cut into them every so often?

In the food colouring test, why must both glasses have the same amount of water and the same number of drops?

Explain why a liquid takes the shape of its container but a solid does not.

Challenge

In a school science lab, a teacher mixes 50 mL of water with 50 mL of alcohol. The total is less than 100 mL (about 97 mL). Use the particle theory to explain where the missing volume went.

Ayesha says, "It's only a theory, so it's probably not true." How would you answer her?

A ping-pong ball has a small dent (but no crack). A grown-up puts it in a bowl of hot tap water, and the dent pops out. Explain why, using two postulates.

Check your understanding

Quiz

Which statement is a postulate of the particle theory?

The spreading of a smell through a room is called...

Food colouring spreads fastest in...

Why can gases be squeezed into a smaller space?

Which postulate explains why solids keep their shape?

In science, a theory is...

For parents and teachers

Big idea: Ontario Grade 7 expectation C2.1 asks students to understand the particle theory of matter. Your child already used a simple particle model in Grade 5; now they learn its postulates and use them to explain observations. The food colouring test practises a fair test (A1.2), safety (A1.4) and communicating with science words (A1.5).

How to help: When something happens in daily life (a smell spreading, a jar lid loosening under hot water, a tire going soft in the cold), ask "Which postulate explains that?" Ask your child to finish the sentence "...because the particles...".

Common difficulties:

Thinking particles stop moving in solids or in the cold. They always move; they just slow down.
Thinking particles themselves get bigger when heated. The particles move faster and the spaces between them grow, so the material expands.
Thinking "theory" means "guess". Explain that a scientific theory is well tested.

Safety: you handle the hot water. Use hot tap water only, not boiling water.