Same atoms, a brand-new thing.

Reactants go in, products come out.

Every reaction has a before and an after, like a recipe. The ingredients you mix together are the reactants — that word just means "the things that react." After the reaction, you're left with the products — the new substances that got made. Chemists love to write this as a little before-and-after story with an arrow in the middle: reactants → products. The arrow simply means "turns into."

And the products are genuinely new substances — not just the old ones moved around or warmed up. That's the whole point of calling it a chemical reaction. When iron turns to rust, the soft orange flakes behave nothing like the shiny silver metal you started with. When you bake a runny batter into a fluffy cake, you can't pour the cake back into the bowl. New stuff, with new properties, has appeared — built from the same atoms, rearranged.

It helps to know what a reaction is not. If you melt an ice cube, you get water — but that's still the same stuff, just softer; the atoms never changed partners. Tear a piece of paper in half and you've still got paper. Dissolve sugar in tea and the sugar is still there, just spread out, ready to taste again. Those are physical changes — shape, size or state shifting around, with no new substance born. A chemical reaction is the deeper kind of change: the atoms themselves find new partners, and something genuinely new comes out the far end of the arrow.

The bricks are never destroyed — just regrouped.

Let's push the LEGO picture a little harder, because it's doing real work here. Every substance is built from atoms — hydrogen, oxygen, carbon, iron, and the rest — locked together by bonds, the chemical "snaps" that hold one atom to another. A water molecule, for example, is two hydrogen atoms snapped onto one oxygen atom. A reaction is the moment those snaps come undone and the atoms click into a different arrangement.

Picture two LEGO models on a table: a little plane and a little boat. To turn them into a single big rocket, you don't run to the shop for new bricks. You pull the plane and the boat apart, and you build the rocket out of the exact same pieces. Maybe a few are left over to start a second small model. Either way, every brick that was on the table at the start is still on the table at the end. Not one appeared from nowhere; not one disappeared into nowhere.

Atoms obey that same rule, perfectly. In an ordinary chemical reaction, atoms are never created and never destroyed — they're only rearranged. So if you began with four hydrogen atoms and two oxygen atoms, you will still have four hydrogen atoms and two oxygen atoms when the reaction is finished, no matter how dramatic the fizzing and flashing looked. They'll just be wearing a different costume — bundled into new molecules.

This is also why nothing is ever truly weightless or "used up." Since the very same atoms are present before and after, the total mass — how much everything weighs — stays the same too. Chemists call this conservation of mass: same atoms in, same atoms out, so the scale doesn't budge. If a reaction looks like it lost weight, it's almost always because a gas floated away unnoticed. Catch that escaping gas in a sealed jar and weigh the lot, and you'll find the mass was there all along.

Watch the bonds break and reform.

On the left, two hydrogen molecules and one oxygen molecule. Press React! The bonds snap, the atoms drift to new partners, and water forms on the right. Keep your eye on the atom count underneath — it never changes. That's conservation, live.

Did you spot the trick? At the start the atoms are grouped as 2 H₂ + O₂ — two hydrogen pairs and one oxygen pair. At the end they're grouped as 2 H₂O — two water molecules. The grouping changed completely, but the headcount didn't: four hydrogen atoms and two oxygen atoms, before and after. The bonds you saw snap apart in the middle are the old "snaps" letting go; the bonds that reappeared on the right are brand-new snaps holding the new molecules together.

Try pressing Reset to start and running it again, watching a different part each time. First time, follow one oxygen atom and see who it ends up bonded to. Next time, watch the moment in the middle when the atoms are briefly free — no bonds at all — drifting between their old partners and their new ones. That free, in-between instant is the heart of every reaction: the old build is gone, the new build hasn't formed yet, and the same bricks are simply floating, waiting to click together a new way.

Four signs a reaction just happened.

Atoms are far too small to see, so how do you know a reaction has taken place at all? You watch for the signs — the clues a reaction leaves behind as it makes new substances. There are four classic ones, and once you know them you'll start spotting reactions all over your kitchen.

The first is a colour change: a new substance often has a new colour, like silvery iron turning orange with rust. The second is gas or bubbles: froth suddenly appearing from a liquid, like the fizz when baking soda meets vinegar — that's a new gas being born. The third is a temperature change: many reactions pour out heat (a hand warmer, a campfire), and a few do the opposite and turn cold (an instant cold pack). And the fourth is light: some reactions give off a glow, like a snapped glow stick. Tap each one below to meet a familiar example.

Try it · demo two — spot the sign

Click a sign to see a real, everyday example of it. Each one is a reaction caught in the act of making something new.

One gentle warning: a sign is a strong hint, not absolute proof. A kettle makes steam and bubbles, but that's just water boiling — a physical change, not a reaction. A light bulb glows hot without any reaction inside it. So detectives look at the whole picture: did a genuinely new substance appear, one you can't simply un-do? When the answer is yes, and one of these signs showed up, you've almost certainly caught a chemical reaction red-handed.

They're happening all around you.

Chemical reactions aren't locked away in laboratories with bubbling flasks. They're in your kitchen, your garden, your party drawer. Here are four you've almost certainly met, each one just atoms finding new partners.

Notice how each one wears its signs on its sleeve. Rusting shows a colour change; burning gives heat and light; baking puffs up with gas and browns; a glow stick shines. Different reactions, same simple story underneath: bonds breaking, atoms regrouping, new substances appearing — and the four signs are just the clues left at the scene.

Some reactions run backwards. Most don't.

Here's a fair question: if a reaction just rearranges atoms, can you always rearrange them back? Sometimes — but usually not easily. Plenty of everyday reactions are a one-way street. You can't un-burn a piece of toast, un-rust a nail, or pour a baked cake back into the bowl. Once the atoms have settled into their new, comfortable arrangement, they're quite happy to stay there, and getting them back would take far more effort than it's worth.

A few special reactions, though, can be coaxed to run both ways — these are called reversible reactions. The clearest everyday example lives in a rechargeable battery: while it powers your phone, a reaction runs in one direction; when you plug it in to charge, electricity pushes that same reaction backwards, resetting the atoms so you can use them all over again. That back-and-forth is exactly why one battery can be drained and refilled hundreds of times.

You don't need to memorise which reactions are which. Just hold the gentle idea that direction matters: some changes lock in for good, while a handful can be wound forwards and backwards like a tape. Either way, the rule from before still holds — the atoms are only ever rearranged, never made or lost.

The myth: "the stuff just disappears."

Watch a log burn down to a little ash and it really feels like most of the wood simply vanished into thin air. And the opposite feels true too: a tiny seed grows into a huge tree, as if matter appeared from nowhere. So it's easy to believe that reactions can destroy stuff or conjure brand-new stuff out of nothing. They can't — and seeing why is the satisfying part.

The burning log is the perfect example. Its atoms mostly combined with oxygen and drifted away as invisible gases — carbon dioxide and water vapour — leaving only a little ash behind. Nothing was destroyed; the wood was rearranged into gases you simply can't see. If you could trap every puff of smoke and weigh it along with the ash and the oxygen that joined in, the total would match what you started with, right down to the gram. Same atoms, same mass — every time. That's the LEGO rule again, dressed up as a campfire.

You be the detective.

Three quick cases. Decide whether a chemical reaction took place — pick an answer and you'll get an instant explanation.