How to Explain States of Matter to Kids

Everything around us is made of matter, and matter usually shows up as a solid, a liquid or a gas. Children can sort things into these groups from an early age. Understanding why they differ takes longer, and that is where good activities help.

The three states in plain words

Solid: keeps its own shape. A spoon, a stone, an ice cube.
Liquid: flows and takes the shape of its container, but stays the same amount. Water, milk, oil.
Gas: spreads out to fill whatever space it is in. Air, steam, the gas in a fizzy drink’s bubbles.

A quick test for young children: “Can I pour it? Can I hold it in my hand and it keeps its shape? Can I see it?”

Why they differ

Everything is made of tiny particles too small to see. In a solid, particles are packed close and held in place, so they can only wiggle. In a liquid, they are still close but slide past each other. In a gas, they are far apart and zoom around.

A simple way to act this out: gather a group, or just two or three children. Stand shoulder to shoulder and wobble (solid). Loosen up and shuffle around while still touching (liquid). Spread out and walk about freely (gas). Add energy by making the movements bigger, and you have shown heating. Take away energy, and you have cooling. Everyone enjoys this, and it requires no equipment.

Changes of state

Melting: solid to liquid, when heated. Freezing: liquid to solid, when cooled. Evaporating: liquid to gas. Condensing: gas to liquid.

Try these with a child:

Ice cube race. Put an ice cube on a plate in the sun and another on a plate in the shade. Which melts first? An adult should supervise any heat source, and a warm room is enough.
Puddle watch. Pour a spoonful of water on a dark paving stone or a plate and draw around it. Check every ten minutes. Where does it go? Our lesson Where does a puddle go? looks at this closely.
Mirror breath. Breathe onto a cold mirror or window and watch it fog. The invisible water vapour in your breath is turning back into tiny drops.
Cold glass. Fill a glass with ice water and leave it. Drops appear on the outside. The water did not leak through. It condensed from the air.

Do not heat liquids with young children. Steam and hot water can cause serious burns, so any heating should be done by an adult at a distance, if at all. The room-temperature experiments above are enough.

Why does ice float?

Most solids are denser than their liquids and sink. Water is unusual: when it freezes, its particles line up in a spread-out pattern, making ice less dense than water. That is why ice floats and why lakes freeze from the top. See Why does ice float? and the post why does ice float explained for kids.

The tricky cases

Children love to ask about things that do not fit neatly.

Sand: each grain is solid, but a pile pours like a liquid.
Cloud and steam: what you see is not gas, it is tiny drops of liquid water. The gas, water vapour, is invisible.
Oobleck (cornflour and water): feels solid when you hit it and runs like a liquid when you let it rest. See oobleck: solid or liquid? and the lesson Is oobleck a solid or a liquid?.
Foam and whipped cream: a mix of gas and liquid.

When your child brings up a tricky example, treat it as a discovery. Say, “Scientists argue about things like that too.”

Is air really something?

A common mix-up is that gases are “nothing”. Show otherwise: blow up a balloon and weigh it against a deflated one on a simple balance, or push an upside-down cup into water and see that the air stops the water entering. Air takes up space and has mass.

Words to learn

particle, solid, liquid, gas, melt, freeze, evaporate, condense, vapour.

Do not force all of them at once. Use whichever fit what you can see.

Questions worth asking

Where does the water go when a puddle dries?
If you put a cold glass in a warm room, why does it get wet outside?
Is toothpaste a solid or a liquid? Why is it hard to decide?

Where to go next

The solids, liquids and gases unit covers these ideas in sequence, and what melts ice fastest: salt, sugar or sand is a neat follow-up experiment. Our parents page explains how lessons are set out for families.