Water can be a rock-hard ice cube, a splashy drink, or an invisible gas floating in the air above your kettle. It's still H₂O the whole time — nothing is added and nothing is taken away.
So what on earth is actually changing? By the end of today, you'll be able to explain exactly what's happening — down to the tiniest particles you can't even see.
Before we start — what do you think makes ice different from steam, if they're both made of the exact same water?
Drag each card below into the column where it belongs — is it usually a solid, liquid, or gas at room temperature? Don't worry about getting them all right — this just helps us see where to start.
🖱️ Drag a card onto the correct column (or tap a card, then tap the column, on touch devices).
Every single thing around you — this screen, the air, your drink — is made of incredibly tiny particles. They're far too small to see, even with most microscopes. Scientists call this idea particle theory.
Particle theory says three important things:
Anything that has mass and takes up space. If you can touch it, weigh it, or bump into it, it's matter.
A very tiny piece of a substance — so small that millions of them fit inside a single grain of sand.
True or False: There is empty space between particles.
Picture an ice cube. It keeps its shape whether it's on a plate or in a cup. Why? It's all about how the particles are arranged and moving.
In a solid, particles are packed tightly in a fixed pattern. They can't swap places — they just vibrate (jiggle) on the spot. Press start and watch them shake without ever leaving their position!
| Property | What happens | Why (particle explanation) |
|---|---|---|
| Shape | Fixed — keeps its own shape | Particles are locked in place and can only vibrate |
| Volume | Fixed | Particles are packed as tightly as possible |
| Compressible? | No — can't be squashed | There is almost no space between particles |
| Movement | Vibrate on the spot only | Strong forces hold particles in fixed positions |
Why can't you squash a solid brick into a smaller shape with your hands?
Pour water from a bottle into a bowl, and it spreads out to fill the bottom of the bowl. Liquids don't keep a fixed shape — but they don't spread out endlessly either. Let's see why.
In a liquid, particles are close together but can slide and roll past each other. That's why liquids flow and take the shape of their container, while still having a fixed amount (volume).
| Property | What happens | Why (particle explanation) |
|---|---|---|
| Shape | Takes the shape of its container | Particles can move around and slide past each other |
| Volume | Fixed | Particles stay close together, just not locked in place |
| Compressible? | Very slightly, but basically no | Particles are still close together, with only tiny gaps |
| Movement | Slide, roll and move around each other | Forces between particles are weaker than in a solid |
Try this before revealing the answer: if you pour the same amount of water into a tall thin glass and then a wide short bowl, does the volume of water change?
Spray a little perfume in one corner of a room, and soon you can smell it everywhere. Gas particles don't just flow like a liquid — they shoot off in every direction, filling all the space available.
In a gas, particles are spread far apart and move fast, in straight lines, in every direction, bouncing off each other and the walls of their container.
| Property | What happens | Why (particle explanation) |
|---|---|---|
| Shape | Fills the entire container | Particles move freely and spread out in all directions |
| Volume | Not fixed — fills any space | There are large gaps between fast-moving particles |
| Compressible? | Yes — easily squashed | Lots of empty space between particles to close up |
| Movement | Fast, random, in every direction | Almost no force holding particles together |
Which state of matter would be easiest to compress (squash into a smaller space)?
Fixed shape
Fixed volume
Vibrate only
Takes container shape
Fixed volume
Slide past each other
Fills container
No fixed volume
Fast & free-moving
Drag a description to the matching state (or the state to the matching description) — or tap a description, then tap the state it matches.
How do you know particles are really moving, if you can't see them? Diffusion is one of the clues. When you open a bag of crisps, the smell doesn't stay trapped in one spot — it spreads through the room.
The movement of particles from an area where there are lots of them to an area where there are fewer of them, until they are evenly spread out. It happens naturally in liquids and gases because particles are always moving.
Orange particles start bunched on the left. Press start and watch them spread out through the container on their own — that's diffusion in action!
Would diffusion happen faster in warm water or cold water?
In 1827, a scientist called Robert Brown looked at tiny pollen grains floating in water through a microscope. He noticed they jittered around randomly — even though nothing was pushing them. Why?
The random, jerky, zig-zag movement of large particles (like pollen or smoke specks) caused by millions of tiny, fast-moving particles (like water or air molecules) colliding with them from random directions.
The big yellow speck is a "smoke particle." The tiny fast dots are invisible air particles. Watch how their random collisions push the big speck in a jittery zig-zag path.
In Brownian motion, what actually causes the big particle to move?
Matter can switch between solid, liquid, and gas. These switches are called changes of state. Nothing new is created and nothing is destroyed — it's a physical change, not a new substance.
Red arrows = heat added Blue arrows = heat removed Purple arrows = direct solid↔gas changes
When a solid is heated, its particles gain energy and vibrate more and more, until they break free of their fixed positions and can slide past each other — it becomes a liquid.
When a liquid is cooled, particles lose energy, slow down, and pack together into a fixed, ordered arrangement — it becomes a solid.
True or False? Melting and freezing happen at completely different temperatures for the same substance.
When a liquid is heated to its boiling point, particles throughout the whole liquid gain enough energy to escape as gas — you see bubbles forming inside the liquid, not just at the surface.
A slower change where only particles at the surface of a liquid gain enough energy to escape as gas. It happens at any temperature, not just the boiling point — just more slowly when it's cooler.
When a gas is cooled, particles lose energy, slow down, and move close enough together to form a liquid again.
| Boiling | Evaporation | |
|---|---|---|
| Where it happens | Throughout the whole liquid | Only at the surface |
| Temperature | Only at the boiling point (e.g. 100°C for water) | Any temperature |
| Speed | Fast | Slow |
| Signs | Bubbles, rapid gas escaping | No bubbles, gradual disappearance |
A wet towel dries on a washing line on a mild day, with no bubbling. Is this boiling or evaporation?
Most changes of state go solid ⇄ liquid ⇄ gas, one step at a time. But a few substances can skip the liquid stage completely!
When a solid is heated and turns directly into a gas, without ever becoming a liquid in between.
The reverse of sublimation: a gas turns directly into a solid, without becoming a liquid first.
True or False? All solids can sublimate easily at everyday temperatures.
Drag the temperature slider and watch water particles change from solid to liquid to gas — and back again. Predict what will happen before you drag!
Watch for the transition points: melting at 0°C and boiling at 100°C (for water, at normal pressure).
Slide the temperature to 50°C. What state is the water in, and why?
Here's a jumbled description of ice being heated until it becomes steam. Drag the cards (using ☰) into the correct order, 1st to 4th — or use the ▲▼ buttons — then press Check Order.
12 questions, getting a little harder as you go. Good luck!
Try explaining to a family member: why does a puddle disappear on a sunny day, without ever boiling? Use the words particles, energy, and evaporation in your answer.
Great work exploring solids, liquids, gases, and how they change into one another. Jump back to any section any time using the map below.