Air Is All Around Us – Matter, Water and Air (Primary 3 Science and Elementary Technology) | Ganzaa

Matter, Water and Air

Lessons

Primary 3 · Science and Elementary Technology

Matter, Water and Air

3 lessons

Lesson 3 of 3

Air Is All Around Us

14 min

Let's begin

Mugisha holds up a plastic bottle with no lid and turns it upside down. Nothing falls out.

"Teacher, this bottle is empty. There is nothing at all inside it."

His teacher smiles and puts a basin of water on the table. "Come and push it into the water," she says. "Then tell me again that it is empty."

Today you will prove that air is real, that it takes up space, and that it is strong enough to push. Then you will build a pinwheel and let the wind spin it for you.

Air is a gas — and it is everywhere

You met gases in the first lesson of this unit. Air is a gas. That means it has no shape of its own, and it spreads out until it fills every bit of space it can reach.

Air is in your classroom. It is in your school bag. It is in the space between you and the friend beside you. It is inside your shoe, and it is inside Mugisha's "empty" bottle. Air has no colour, so you see straight through it and think there is nothing there.

You cannot see air — but you can feel it. Take your exercise book right now and fan it fast in front of your face.

Felt that? That cool push on your cheek is air. It is real. You just met it.

Test 1: Air takes up space (the bottle in the basin)

Worked example

Fill a basin with water. Take a bottle with nothing in it and no lid. Push the bottle straight down into the water with the open mouth pointing down.

What do you see? Bubbles come rushing up out of the bottle.

Now think slowly and carefully. Bubbles cannot come out of a bottle that has nothing inside it. Something came out. Something was in there.

That something was air. The bottle was never empty — it was full of air. And notice the order of what happened: the water could only get in after the air got out.

Air takes up real space. Space that water has to fight it for.

Test 2: The dry paper in the cup

This one surprises everybody. You need a cup, a basin of water and a small ball of dry paper.

Squash the paper down into the bottom of the cup so it is stuck tight and cannot fall out.
Turn the cup upside down. The paper stays where it is, up at the top of the upside-down cup.
Push the cup straight down into the water, holding it perfectly level. Do not tilt it! Push it right under the surface.
Count slowly to ten. Then lift the cup straight up, still perfectly level.
Take out the paper and feel it.

The paper is dry. Bone dry, after being underwater.

Why? The cup was full of air. When you pushed it under, the air had nowhere to escape to, so it stayed inside and held the water out. The water simply could not reach the paper, because air was already sitting in that space.

Now do the whole thing again — but this time, while the cup is under the water, tilt it.

Bloop! A big bubble of air escapes and races to the surface. And in the very same moment, water rushes in to take the space the air left behind. Lift the cup out and the paper is soaked.

There is your proof, twice over. Where there is air, water cannot be. Air takes up space.

Test 3: Air pushes back

Blow into a plastic bag and twist the top shut so nothing escapes. Now squeeze it hard.

Feel that? It pushes back on your hand. The air is squashed inside and it presses out on every side.

That is the same air that makes a football hard enough to kick and a bicycle tyre firm enough to ride on. A flat tyre and a full tyre are made of exactly the same rubber. The only difference between them is the air inside.

Moving air is called wind, and wind can push

Air standing still is quiet, and easy to forget about. But when air moves, you notice it at once — because moving air pushes.

Moving air has its own name: wind.

Worked example

Put a ball of paper on your desk. Blow gently at it: it wobbles a little. Now blow really hard: it rolls right off the desk.

Your breath is moving air. Moving air is wind. The wind pushed the paper — and the harder the wind, the bigger the push. That last part matters. Remember it.

Look for the wind at work around you:

It pushes the wet clothes on the line so they flap, and they dry faster because of it.
It pushes dust and dry leaves along the road.
It pushes a sail, and a boat crosses Lake Kivu with no engine and no paddle at all.
Farmers toss beans or sorghum up out of a flat basket. The wind is strong enough to carry the light husks away, but not strong enough to carry the heavy grain — so the clean grain falls back into the basket. Very clever.
A strong wind can bend a whole tree, and a really strong one can lift the iron sheets off a roof.
Huge machines called wind turbines have long blades. The wind pushes the blades round and round, and that spinning is used to make electricity.

You never see the wind itself. You only ever see what it pushes. That is enough.

Build a paper pinwheel and catch the wind

Now you will build something that turns the wind's push into a spin.

You need: a square of paper about the size of your hand (about 15 cm on each side), scissors, a pin or drawing pin, and a pencil with a rubber on the end (or a thin stick).

With a ruler, draw a line from each corner of the square to the corner opposite it. The two lines cross in the middle and make a big X.
Cut along each line, starting at a corner and stopping about two fingers' width (roughly 2 cm) before the middle. Do NOT cut all the way to the middle, or your square will fall apart into four pieces.
Look at the paper. Around the edge you now have eight points. Take any one point and bend it into the middle — bend it, curve it, do not fold it flat. Hold it there with your thumb.
Now skip the next point, and bring the one after it into the middle as well. Keep going round, bringing in every second point, until you are holding four points together in the middle. The paper now looks like a flower.
Push the pin through all four points and through the middle of the square, then push the pin into the rubber on the end of the pencil. Do not push it in tight — leave a small gap, so the paper can turn freely.
Hold the pencil and blow at the pinwheel. Then take it outside and hold it up into the wind.

It spins!

Why does it spin? Look at your pinwheel from the side. Every bent corner has made a slanted face, like a tiny sail. When moving air hits a slanted face it cannot go straight through, so it slides off to the side — and as it slides, it pushes the paper sideways. All four faces are slanted the same way, so all four get pushed the same way round. The whole wheel turns.

Now test it like a scientist. Blow gently: it turns slowly. Blow hard: it spins fast. A stronger wind gives a bigger push, so the pinwheel spins faster. Exactly the same rule as the paper ball on your desk.

If it will not turn: the pin is almost certainly pushed in too tight, or your four points got squashed flat instead of staying curved. Loosen the pin, puff the corners back out, and try again.

Warning

Never put a plastic bag over your face or your head, not even for one second. You need air to breathe. Do the bag test with your hands only, far away from your face.

Pins and scissors are sharp. Ask an adult to help you with the cutting and the pin.

Tip

Two things to remember about air:

Air takes up space — the bubbles from the bottle, and the paper that stayed dry in the cup.
Moving air pushes — your pinwheel, the clothes on the line, the sail on the lake.

Air standing still = air. Air on the move = wind. Harder wind = bigger push.

✦

Try it yourself

Answer in your head first, then tap Show answer to check.

You push an "empty" bottle mouth-down into a basin and bubbles rush out of it. What does that prove?

Answer: the bottle was never empty — it was full of air. Air takes up space, so the water could only get in once the air got out.

Mugisha pushes an upside-down cup with dry paper inside it straight down under the water, holds it level, then lifts it straight out. Is the paper wet or dry, and why?

Answer: dry. The cup was full of air, and the air had nowhere to go, so it held the water out and kept the paper dry.

What happens if he tilts the cup while it is under the water?

Answer: a big bubble of air escapes to the surface, water rushes in to take the space it left, and the paper gets soaked.

Uwase's pinwheel is turning very slowly. She wants it to spin faster. What can she do?

Answer: blow harder at it, or take it out where the wind is stronger. A stronger wind gives a bigger push, so the pinwheel spins faster.

Name three things the wind can push.

Answer: many answers are right. Clothes on the line, dust, dry leaves, a sail on Lake Kivu, the light husks off the grain, the blades of a wind turbine, the branches of a tree — and your pinwheel.

Quick check

Bubbles rush out of an "empty" bottle pushed into water. Why?

Note

What you learned today:

Air is a gas. It is all around you, even inside a bottle that looks empty.
Air takes up space: bubbles escape from a bottle pushed into water, and paper in an upside-down cup stays dry.
Squashed air pushes back — that is what makes a football hard.
Moving air is called wind, and wind can push things.
The stronger the wind, the bigger the push — so your pinwheel spins faster.

1Air is a gas — and it is everywhere

2Test 1: Air takes up space (the bottle in the basin)

3Test 2: The dry paper in the cup

4Test 3: Air pushes back

5Moving air is called wind, and wind can push

6Build a paper pinwheel and catch the wind