Air, Wind and Sound
Primary 4 · Science and Elementary Technology
Air, Wind and Sound
Lesson 1 of 5
15 min
Let's begin
Mugisha pumps up his bicycle tyre. The tyre gets fatter and harder, yet he cannot see anything going in. "What is inside my tyre?" he wonders.
The answer is air. You cannot see it, but you can prove it is there.
Air is a mixture of gases that covers the whole Earth. We cannot see it, because clean air has no colour. Clean air also has no smell and no taste. But air is matter, just like water and stones, so it has these properties:
Push a crumpled piece of paper into the bottom of a dry cup. Turn the cup upside down and push it straight down into a basin of water. Lift it out. The paper is dry! Air trapped in the cup took up the space, so the water could not get in.
A balloon gets bigger when you blow air into it, because the air needs space. Air has no fixed shape: it spreads out to fill whatever container it is in.
Hang two equal balloons, one blown up and one empty, from the two ends of a stick balanced on a string. The side with the full balloon goes down: the air inside it has weight.
Air is made of tiny particles that are always moving fast. They bump against everything they touch. All these bumps together make air pressure. Air pressure keeps a tyre or a football hard. When you suck on a straw, air pressure pushes the juice up into your mouth.
Close the end of a syringe full of air with your finger and push the plunger. It moves in, because there are big spaces between the air particles. Try the same with a syringe full of water: it hardly moves.
Pull a balloon over the neck of an empty bottle and stand the bottle in warm water. The balloon slowly fills up! Heat makes the air particles move faster and spread out, so the air expands. When it cools, it contracts again and the balloon goes flat.
Moving air is called wind. You can feel it on your face and see it shake the banana leaves.
Tip
Air SPWEM: air takes up Space, Presses, has Weight, Expands when hot and Moves.
Try each one first, then tap Show answer to check.
Why does the paper stay dry in the upside-down cup?
Answer: Air trapped in the cup takes up space, so the water cannot get in.
A football left in the hot sun becomes very hard. Why?
Answer: The air inside is heated and expands, so it presses harder on the inside of the ball.
What keeps a bicycle tyre hard?
Answer: Air pressure: the air particles inside push on the tyre.
Name two properties of clean air you cannot sense with your eyes or nose.
Answer: It has no colour and no smell (and no taste).
What you learned today
Sources and credits
The exercises (Try it yourself and the practice questions) are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
Quick check
Match each experiment to what it shows about air.
Tap one on the left, then its partner on the right.
