Stir sand into water, dissolve salt, blend two liquids, dab a black pen — and you've made a mixture. The lovely secret of mixtures? Nothing changes inside them, so you can always pull the parts back out. You just need the right tool.
Open the toolkitWhen you stir two things together into a mixture, neither one becomes something new. The salt is still salt; the water is still water. They're just sharing space. And anything that's only sharing space can be un-shared — pulled cleanly apart again.
That's the promise behind every separation method in the lab. You're never "unscrambling an egg." You're sorting a drawer where the socks and the pencils got jumbled together — fiddly, maybe, but completely possible, because each thing is still itself. The whole skill is choosing the move that tells the parts apart. This page hands you a small toolkit of four such moves, and a simple way to know which one to reach for.
A mixture is two or more substances jumbled together without reacting — without turning into something brand new. Picture a bowl of trail mix: raisins, nuts and chocolate chips all tumbled in one bag. They touch, they share the bag, but a raisin is still a raisin and a peanut is still a peanut. If you were patient (and hungry), you could pick every piece back out by hand.
That's the difference between a mixture and a compound. In a compound, the substances have actually joined up and changed — like hydrogen and oxygen bonding into water — and you can't pull them apart with tweezers, a sieve or a warm afternoon; it takes a real chemical reaction. Separation techniques are the easy, peaceful kind of un-mixing, and they only work because a mixture never truly changed in the first place.
So here's the trick the whole topic turns on: every part of a mixture keeps its own properties. Sand stays solid and heavy. Salt stays able to dissolve. Each liquid keeps its own boiling temperature. Each dye keeps its own colour and "stickiness." Those leftover differences are exactly the handles we grab. A separation method is really just a clever way of saying: "You two are different in this one way — so off you go, each to your own side."
Each tool below grabs a different difference between the parts of a mixture. That's the secret to picking the right one: don't ask "what's the fanciest method?" — ask "in what way are these two things different, and which tool notices that?" Here are all four, with the exact property each one exploits.
Pour the mixture through paper full of tiny holes. Bits bigger than the holes (undissolved solids) are trapped on top; the liquid slips through. Catches the chunky from the runny.
Gently heat a dissolved solid in liquid. The liquid escapes into the air as vapour (an invisible gas), but the solid can't — so it's left behind as dry crystals. Keeps the part that stays.
Like evaporation, but you catch the vapour and cool it back into a liquid in a fresh container. Two liquids boil at different temperatures, so one rises first — you collect it pure. Keeps the part that flies off.
Let a liquid creep up paper, carrying a blob of mixed colours with it. Each hidden substance travels at its own speed, so they spread into separate bands. Sorts a blend by how fast each part moves.
Read those four "Property" tags again — particle size, one part leaves as vapour, boiling point, travel speed — and you've basically learned the topic. Everything else on this page is just practising the question: which difference does this mixture hand me?
Pick a mixture to "pick it up," then tap the technique you'd use. If it fits, the card locks in green and tells you why. If not, you'll get a nudge and can try again. Match all six to clear your toolkit.
Tap a mixture above to start. We'll ask you to pick the tool that separates it.
Notice what your brain is really doing each round. You're not memorising "salt water goes with evaporation" like a phone number. You're asking what makes the two parts different — is one an undissolved lump? is one dissolved? are they two liquids? is one secretly many colours? — and the right tool falls out of the answer. That question is the whole game.
Here's chromatography in slow motion. A single spot of "black" ink sits on a strip of paper dipped in water. Press Run and watch the water climb — carrying the ink's hidden dyes up with it, each at its own speed, until the one black spot fans out into separate colours.
A single black spot, ready to climb. Press Run to start the water rising.
Why does black split apart at all? Because that "black" was never one substance — it's a blend of coloured dyes mixed so thoroughly they fool your eye into seeing one colour. As the water (called the solvent — the liquid that does the carrying) soaks upward, each dye is dragged along, but some cling to the paper more and some ride the water more eagerly. The eager ones race ahead; the clingy ones lag behind. By the time the water nears the top, every dye has landed at its own height — a tidy little column of the colours that were hiding inside the black all along.
That "travels at its own speed" idea is more powerful than a pen trick. The same method helps scientists check which colourings are in a food, spot whether a banknote's ink is genuine, and tell apart the substances in a sample too tiny to see. One spot in, a row of answers out.
When a mixture lands in front of you, run down this little ladder of questions. Stop at the first one that's a "yes" — that's your tool. It works because each question is really asking, "is this the difference I can grab?"
One mixture can even answer "yes" to more than one question, depending on what you want out of it — and that's the next, juicier idea.
Theory is nice; let's actually do two separations the way you would in a lab. Watch how the goal — which part do I want? — decides the tool.
The mud is undissolved solid — the easiest case of all.
Look first. The water is cloudy and grainy; you can see specks of soil drifting and settling. That's a sign the solid is insoluble — it never dissolved, it's just floating about. Undissolved solid means question one is a "yes."
Set a folded paper filter inside a funnel over an empty beaker, and pour the muddy water through.
The soil specks are far too big to fit through the paper's holes, so they stay trapped on top — that leftover solid is called the residue. The clear water that drips through is the filtrate. Job done with one tool.
Same mixture, but now you want two different things — so you'll reach for two different tools.
First, notice filtering is useless here. The salt is dissolved — broken into pieces far too tiny to catch — so it would sail straight through the paper with the water. (More on that trap in a moment.)
To keep the salt: gently heat the salt water in a dish. The water escapes as vapour and drifts off into the air; the salt can't turn to vapour, so it's stranded behind as dry crystals. That's evaporation — you kept the part that stays.
To keep the water instead: heat the salt water again, but this time lead the rising steam into a cool tube where it turns back into liquid and drips into a separate cup. That recovered liquid is pure water; the salt is left behind in the first flask. That's distillation — you caught the part that flew off.
Same starting mixture, two goals, two tools. The mixture was happy to give back either part — because, remember, the salt and water never stopped being themselves.
This is the deepest lesson of separation: the method follows the goal, not just the mixture. "Salt water" doesn't have one right answer carved in stone. Want the salt? Evaporate. Want the water? Distil. Decide what you're after first, and the tool almost chooses itself.
It feels so reasonable. Filtering pulls solids out of liquids, salt is a solid, so surely a fine enough filter would catch the salt and leave you with pure water? It's the single most common slip in this whole topic — and it quietly misunderstands what dissolving actually does.
Salt is a solid, so a filter with small enough holes will trap it on the paper and let clean water drip through — just like catching sand.
When salt dissolves, it breaks into pieces so unbelievably tiny they spread evenly all through the water, making a clear solution. Those pieces are far smaller than any hole in filter paper, so they slip straight through with the water. The filter catches nothing — to remove dissolved salt you must evaporate or distil.
Here's the clean rule that kills the confusion forever: filtering only catches what never dissolved. Sand, mud, chalk, coffee grounds — solids floating about as visible bits — those it grabs beautifully. But the moment a solid dissolves and the liquid turns clear, the filter is blind to it. Clear-and-dissolved is a job for heat (the vapour tricks), never for paper. Test yourself: if you can still see the solid, filtering might work; if the liquid has gone perfectly clear, it won't.
Three fast calls to lock it in. Pick an answer for an instant explanation.
Pick one to check your thinking.
Pick one to check your thinking.
Pick one to check your thinking.
Every part of a mixture keeps its own properties — so it can always be pulled back out.
Particle size → filter. Stays behind → evaporate. Boiling point → distil. Travel speed → chromatography.
Ask which part you want first; the right method almost chooses itself.