Matter, Water and Air
Primary 3 · Science and Elementary Technology
Matter, Water and Air
Lesson 1 of 3
14 min
Let's begin
Gatete's teacher puts three things on the table. A stone. A cup of milk. And a plastic bag that looks completely empty.
"Two of these you can see and touch," she says. "The third one looks like nothing at all. But I promise you, there is something inside that bag."
She opens the bag, swings it fast through the air and twists the top shut. It puffs up, fat and round, like a little pillow. Today you will find out what filled it — and why scientists put every single thing in the world into just three groups.
Look around you right now. Your desk. Your pencil. The water in your bottle. The air you are breathing in and out.
All of it is made of something. Scientists have one word for that something: matter.
Matter is anything that takes up space and has weight. Your shoe is matter. A drop of rain is matter. Even the air in the room is matter — and before this lesson ends you will see the proof.
Matter comes in three kinds. We call them the three states of matter: solid, liquid and gas. Let us meet them one at a time.
Watch Keza with a small stone.
She drops the stone into a round cup. Shape of the stone? Same as before. She tips it out onto a flat plate. Shape? Exactly the same. She holds it in her hand, then puts it in her pocket. Still exactly the same stone shape.
The cup did not change the stone. The plate did not change the stone. The stone brought its own shape with it everywhere it went.
That is the rule: a solid has its own shape, and it keeps that shape wherever you put it. You can pick a solid up in one piece.
Solids you know: a pencil, a book, a spoon, a brick, a nail, a bar of soap, a bean seed, a football, your desk, ice.
Careful — a solid does not have to be hard. A sponge is soft. A ripe banana is soft. Squeeze them and they squash. But let go, put them down anywhere you like, and each one still has its own shape. They are solids too.
Mugisha pours sugar out of the packet into a cup. It flows in, exactly like water, and takes the shape of the cup.
"So sugar is a liquid!" he says.
He is wrong, and here is how you can show it. Pick up one grain of sugar and look at it very closely. It is a tiny hard lump with its own little shape. Put it in the cup — same shape. Tip it onto the table — same shape. That one grain behaves exactly like Keza's stone.
Sugar pours because there are thousands of tiny solids sliding over one another, like a crowd of people pushing through a gate. The pile changes shape. Each grain never does.
So the test is: look at ONE piece. If one piece keeps its own shape, it is a solid. Sugar, salt, sand, rice, beans and flour all pass that test. They are all solids.
Now watch Keza's cup of milk.
In the round cup, the milk is a round column. She pours it into a flat plate: now it is wide and flat and thin. She pours it into a narrow bottle: now it is a tall thin column.
Same milk. Three completely different shapes.
Here is a question that catches people out. In the flat plate the milk looked much bigger and wider. Was there more milk in the plate?
No! Pour it back into the cup and it fills the cup up to exactly the same line as before. Not one drop was added. Only the shape changed, never the amount.
That is the rule: a liquid has no shape of its own. It takes the shape of whatever is holding it. It pours, it flows, and when it settles it makes a flat, level top — look at a cup of water from the side and you will see that flat surface.
Liquids you know: water, milk, cooking oil, tea, juice, paint, honey. Honey is thick and slow, but tip the jar and it still flows and still takes the jar's shape. It is a liquid.
Spill a liquid on the floor and it cannot hold itself up. It spreads out into a puddle.
Back to the teacher's bag. It looked empty. She swung it through the room, twisted it shut, and it went fat and round. Something filled it.
Squeeze that fat bag. Feel it push back against your hand. There is definitely something in there — and that something is air. The air was in the room all along, and the bag simply caught some of it.
Here is a second proof. Take a bottle with nothing in it, no lid. Push it straight down into a basin of water, with the open mouth pointing down.
Bubbles come rushing out.
Bubbles cannot come out of a bottle that has nothing inside. So the bottle was never empty. It was full of air, and the water pushed the air out.
That is the rule: a gas has no shape of its own and no size of its own. It spreads out until it fills the whole container. Blow a little air into a balloon and the air does not sit in the bottom — it goes everywhere inside, right to the far end, and pushes the balloon out in every direction.
Gases you meet every day: the air you breathe, the air in a football, the steam rising off a boiling pot, and the smell of beans cooking that reaches every room in the house — that smell travels because a gas spreads out.
Most gases have no colour, so you cannot see them. Never think that means they are not there.
Stuck? Ask these three questions and any material in the world will tell you what it is.
| Ask this | Solid | Liquid | Gas |
|---|---|---|---|
| Does it have its own shape? | Yes | No | No |
| Does it flow or pour? | No | Yes | Yes |
| Does it fill the whole container? | No | No, it sits at the bottom | Yes, right to the top |
| Example | A stone | Milk | Air |
Worked example
Is honey a solid or a liquid? Ask the three questions.
All three answers point to a liquid: it has no shape of its own, it flows, and it does not fill the whole jar. Honey is a liquid — a thick, slow, sticky one, but a liquid.
Water is a show-off. It can be a solid, a liquid AND a gas.
You see this every day without noticing. After the rain there are puddles all over the yard. By the afternoon they are gone, and nobody drank them. The sun turned that water into a gas. It is the same reason wet clothes on the line become dry.
And it works backwards too. Cool a gas down and it turns back into a liquid: that is why little drops appear on the underside of the pot lid, and on the outside of a cold bottle.
Heat pushes water on: ice to water, water to gas. Cold brings it back again.
Tip
When you are not sure, ask the shape question:
Say it out loud until you own it: Solid stays. Liquid shapes. Gas escapes.
Work each one out in your head first, then tap Show answer to check.
Uwase puts a bar of soap in a bowl, then moves it onto a plate. Is soap a solid, a liquid or a gas?
Answer: a solid. The bowl and the plate did not change it. It brought its own shape with it.
Cooking oil is poured from a bottle into a flat frying pan, and it spreads out wide and thin. Which state of matter is cooking oil?
Answer: a liquid. It flows, and it took the shape of the pan. It has no shape of its own.
Mugisha says salt must be a liquid, because it pours out of the packet just like water. Is he right?
Answer: no. Look at one single grain of salt: it is a tiny hard lump that keeps its own shape wherever you put it. Salt pours because thousands of tiny solids slide over each other. Salt is a solid.
Keza blows air into a balloon. The air does not stay near the opening — it goes all the way to the far end and pushes the balloon out on every side. Which state of matter is air, and how do you know?
Answer: a gas. Only a gas spreads out to fill the whole container it is in.
An ice cube is left on a plate in the hot sun. An hour later there is a small pool of water on the plate. What happened, and what state is the water in now?
Answer: the ice melted. Heat changed the solid ice into liquid water. If you leave the plate in the sun even longer, the sun will turn that liquid into a gas and the plate will end up dry.
Note
What you learned today:
Quick check
Match each rule to its state of matter.
Tap one on the left, then its partner on the right.
