Water pours and air spreads, but a stone stays put. What is different inside them?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Draw the particles in a solid, a liquid and a gas
Use the particle model to explain why liquids flow and gases spread
A stone keeps its shape. Water takes the shape of its cup. Air fills a whole room. Now we look inside them, at things far too small to see.
A grain of salt, and the particles inside it. They are far too small to see.
Everything is made of tiny bits called particles. A grain of salt holds more of them than there are people on Earth.
The particle model is a way of picturing them. We draw each particle as a small circle. The model explains how solids, liquids and gases behave.
Draw it first
Draw the particles in three boxes
1 Draw a box. Fill it with circles in neat rows, all touching. Label it solid.
2 Draw a second box with an open top. Fill the bottom with circles that touch but are jumbled. Add a flat top and label it liquid.
3 Draw a third box with a lid. Put a few circles in it, far apart. Add short lines behind them. Label it gas.
In a solid, the particles touch and pull on each other. Each one is held in its place. It cannot move around, but it can vibrate. To vibrate is to shake to and fro in a tiny space.
A particle in a solid shakes to and fro. Its neighbours hold it in place.
In a liquid, the particles still touch and still pull on each other. But they are not held in place. They slide past each other, so a liquid can flow.
In a gas, the particles are far apart. They hardly pull on each other. They fly about in every direction. Between them there is nothing at all, only empty space.
The particles are the same in every state. A particle of water is the same in ice, in water and in steam. Only the way the particles are held, and the way they move, is different.
The same particles of water in three states.
Now we can explain what you see. A solid keeps its shape because its particles are held in place. A liquid pours because its particles slide past each other.
A gas spreads out because its particles fly about and nothing holds them. They keep going until they fill the whole container.
See it three ways
With things
Think of people. In a packed queue, each person keeps a spot and only shuffles. In a busy market, people slide past each other. In a big empty field, a few people walk in any direction.
As a picture
In words
In a solid, the particles touch and vibrate in place. In a liquid, they touch and slide. In a gas, they are far apart and move freely.
Try it
Be the particles with beans
You need: a tray or a shallow box, about 30 dried beans or small stones.
Pack the beans into the tray so that they all touch. Shake the tray very gently. The beans only wobble.
Tip the beans into a loose heap at one end. Shake the tray a little more. The beans slide past each other.
Take out all but 5 beans. Shake the tray hard. The beans scatter and fly about.
What you will see: Packed beans wobble in place, like a solid. Loose beans slide, like a liquid. A few beans fly about and fill the tray, like a gas.
If you cannot try it, this is what happens.
A wrong idea many people have
In a solid, the particles do not move at all.
Why it feels right: A stone sits still and keeps its shape.
The test that breaks it: Think of a steel rail in the sun. It gets a little longer as it warms. If its particles stood still, warming could not change its length. The shaking is too small to see or feel, but we feel it as warmth.
What is true: The particles in a solid are held in place, but they vibrate all the time. They never stand still.
Key words
particles
the tiny bits that everything is made of.
particle model
a picture that shows particles as circles and explains how matter behaves.
vibrate
shake to and fro in a tiny space.
flow
move along because the particles slide past each other.
Back to your guess
The answer is C: Inside a stone the bits are held in place. In water they slide. In air they fly about.
Every material is made of tiny particles. In a stone they are held in place. In water they slide past each other. In air they fly about.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
In which state are the particles far apart and flying about in every direction?
See the answer
Gas
The particles of a gas are far apart. Almost nothing holds them, so they fly about and fill the container.
If you chose A: In a solid the particles touch and are held in place. Go to S3.5.1.
If you chose C: In a liquid the particles touch. They slide past each other, but they stay close. Go to S3.5.1.
You can pour water but not a stone. Which reason is right?
See the answer
The particles of water slide past each other. The particles of a stone are held in place.
Sliding particles let a liquid flow. Particles held in place keep a solid in its shape.
If you chose A: The particles are the same in every state. Their size does not explain pouring. How they are held does. Go to S3.5.1.
If you chose B: In a liquid the particles do touch. They are free to slide, and that lets it pour. Go to S3.5.1.
Gas fills a closed jar. What is there between its particles?
See the answer
Nothing at all. It is empty space.
The particles of a gas are far apart. The space between them has nothing in it.
If you chose A: The particles are the gas itself. Between them there is empty space. Go to S3.5.1.
If you chose C: A gas is not a liquid. Its particles are far apart with nothing between them. Go to S3.5.1.
Tariq says, "The particles in a stone do not move." What should you tell him?
See the answer
They vibrate in place. They cannot move around, but they never stand still.
Held in place does not mean still. Each particle shakes to and fro where it is.
If you chose B: The particles of a solid vibrate. You feel this as warmth, and it grows when the solid is heated. Go to S3.5.1.
If you chose C: Gas particles fly about freely. Solid particles only vibrate in place. Go to S3.5.1.
You draw a box of gas with 6 circles. Which drawing is right?
See the answer
Six circles far apart, spread through the whole box.
The particles of a gas are far apart and fill the box. The empty space between them stays empty.
If you chose B: That is how a solid is drawn. The particles of a gas are far apart. Go to S3.5.1.
If you chose C: That is how a liquid is drawn. A gas spreads out to fill the whole box. Go to S3.5.1.
Keep it
Copy this into your exercise book.
All things are made of tiny particles. In a solid they touch and vibrate in place. In a liquid they touch and slide past each other. In a gas they are far apart and fly about.
Draw: Draw three boxes. Put neat rows of circles in the first. Put a jumbled pool in the second. Put a few circles far apart in the third.
