Reversible and Irreversible Changes

Squash a ball of playdough flat and you can roll it straight back into a ball. Melt an ice cube into a puddle and you can pop the water in the freezer and get your ice cube back. But toast a slice of bread until it's brown and crispy… and no matter what you do, you can never turn it back into soft white bread again.

That is the whole of this page in one idea. Some changes can be undone — we call those reversible. Some changes make something brand new and can never be undone — we call those irreversible. Learning to spot which is which turns you into a bit of a scientist.

Reversible: you can get it back

In a reversible change, the stuff is rearranged but it is still the same stuff — so there is always a way to run the change backwards and get the original back. You already met a whole family of these when you learned about changing state:

Melting and freezing. Ice melts to water; freeze the water and the ice comes straight back.
Heating and cooling. Warm chocolate goes runny; let it cool and it sets hard again.
Evaporating and condensing. Water dries into the air; cool that air on a cold window and the water reappears as drops.
Dissolving. Stir salt or sugar into water and it seems to vanish — but it's still there, and letting the water evaporate leaves the salt or sugar behind.
Squashing and stretching. Stretch an elastic band or bend a paperclip, then let go — springy things ping back to their old shape.
Mixing then separating. Sieve stones out of sand, filter mud out of water, or pull iron nails out of a pile of rice with a magnet — the bits were only jumbled together, never changed.

Notice the pattern: nothing new is ever made. The ice, the salt, the elastic band — the same material is still there the whole time, just melted, dissolved or stretched. That is the secret signal of a reversible change.

Irreversible: something new is made

An irreversible change is different in a big way: it makes a new substance, and once the new stuff exists you cannot easily get the originals back. Here are the classic ones:

Burning. Set fire to wood or paper and you get ash, smoke and heat. The ash is not wood any more, and no amount of cooling turns the smoke back into your log.
Cooking. Fry an egg and the runny clear part turns solid and white forever. Bake a cake and the eggs, flour and sugar become something completely new. You can't un-cook them.
Rusting. Leave an iron nail out in the damp and, over weeks, a flaky orange coat creeps over it. That rust is a new substance made from iron, air and water — the shiny nail is gone.
Mixing that fizzes. Pour vinegar onto bicarbonate of soda and it foams up like a volcano. That fizz is a new gas bubbling out — proof that a brand-new substance has been made.
Curdling. Squeeze lemon juice into fresh milk and it splits into lumpy curds and watery whey. You can't stir it back into smooth milk.

Irreversible changes often leave a clue that new stuff has appeared: a colour change (silver nail to orange rust), a new gas given off (the fizz, the smoke), or heat and light pouring out (a flame). Spot one of those and you are almost certainly watching a change that can't be undone.

Two questions to tell them apart

When you meet any change, you can work out which sort it is by asking just two questions:

Can I get the original back? If yes — like water back from ice — it's reversible.
Has something new been made? If yes — a new gas, a new colour, ash where there was wood — it's irreversible.

The two questions are really the same question asked twice. If it's still the same stuff, you can undo it. If it's turned into new stuff, you can't. Try it on the changes below and see the arrow run backwards — or refuse to.

Try it: run the change backwards

Choose an everyday change. The top arrow shows the change happening. The bottom arrow tries to run it backwards — for a reversible change it works and you get a green tick, but for an irreversible change it gets crossed out, because the new stuff simply won't turn back. Read the verdict at the bottom each time.

These three trip almost everybody up:

Dissolving is reversible — the sugar didn't disappear. When sugar "vanishes" into your tea it looks gone forever, but it's still right there, spread out through the water (that's why the tea tastes sweet!). Leave the water to evaporate and the sugar comes back as a sticky layer. Nothing new was made.
Melting is not the same as dissolving. Melting needs heat and turns a solid into a liquid of the same stuff (ice → water). Dissolving needs a liquid to mix into and hides one stuff inside another (sugar → sugary water). Both are reversible, but they are different tricks — don't muddle them.
Cooking and burning make new stuff, so they can't be undone. A cooked egg or a slice of toast is a brand-new substance. Cooling it down does nothing — there is no "un-cook" and no "un-burn". That's the sure sign of an irreversible change.

A baker cracks eggs, tips in flour and sugar, stirs up a gloopy batter and slides it into a hot oven. Twenty minutes later out comes a golden, spongy cake. Now here's the puzzle: you could un-stir the batter if you were very careful — but you can never un-bake the cake back into eggs and flour. Why not?

Because the heat of the oven did something the stirring didn't: it made the ingredients react and join into brand-new substances that simply weren't there before. Baking isn't just mixing, it's changing — that's why it's irreversible. Compare that with getting salt back from seawater: pour salty sea water into a shallow tray, leave it in the sun, and as the water quietly evaporates it leaves behind glittering salt crystals — the very salt that was dissolved in it all along. Dissolving hid the salt; evaporating handed it straight back. One change makes new stuff and locks the door; the other only rearranges, and the door swings both ways.