Separating mixtures, but bitesize.

Part 1What "pure" really means

In everyday life, "pure" can mean clean, or natural, or healthy. "Pure orange juice", "pure mountain water". But in chemistry, pure means something very specific and quite strict.

A pure substance is made of only one type of substance — nothing else mixed in. Pure water is only water particles. Pure copper is only copper. Nothing has been added.

A mixture is the opposite: it contains two or more different substances that are simply jumbled together. The substances in a mixture are not chemically joined — they're just sharing the same space. Because of that, they can be separated again using the techniques in this booklet.

So "pure" mountain water from the advert isn't pure in the chemistry sense at all — it has minerals dissolved in it. It's a mixture.

Part 2The melting-point purity test

Here's the clever bit. You can't always tell if something is pure just by looking. But there's a test: melting and boiling points.

A pure substance has a sharp, fixed melting point and boiling point. Pure ice melts at exactly 0 °C. Pure water boils at exactly 100 °C. Every time. The temperature stays still while it changes state.

A mixture melts and boils over a range of temperatures, not at one fixed number. Salty water doesn't boil at exactly 100 °C — it boils higher, and over a spread. So if a substance changes state over a range instead of at a sharp point, you know it's impure — it's a mixture.

This is genuinely how chemists check purity: measure the melting point and see if it's sharp.

Part 1Solute, solvent, solution

When you stir sugar into tea, the sugar seems to vanish. It hasn't gone anywhere — it has dissolved. The sugar particles have spread out evenly through the water, too small to see. To talk about this properly, chemists use three words that go together:

The solute is the substance that dissolves — the sugar or salt. The solvent is the liquid that does the dissolving — usually water. The solution is what you end up with — the clear, even mixture of the two.

An easy way to remember the order: a soluTE dissolves into a solvenT to make a soluTION. The solute is the one that disappears; the solvent is the one doing the work.

Part 2Soluble, insoluble & saturated

Not everything dissolves. Salt and sugar dissolve in water — we say they are soluble. Sand and chalk don't dissolve no matter how hard you stir — they are insoluble. (This matters in the next topic, because filtration only works on insoluble solids.)

Even a soluble solid has a limit. Keep adding salt to water and stirring, and eventually no more will dissolve — extra salt just sits at the bottom. The solution is now saturated: it holds as much dissolved solute as it possibly can at that temperature.

Part 3Dissolving is not melting — and mass is conserved

Two ideas pupils mix up. First, dissolving is not the same as melting. Melting needs heat and turns a solid into its own liquid (ice → water). Dissolving doesn't need heat — the solid spreads out into a different liquid and the solid is still there, just hidden.

Second, and really important: when something dissolves, the mass is conserved. The solute hasn't been destroyed — its particles have just spread out among the water particles, too small to see. So the total mass stays exactly the same.

Try the maths: 100 g of water plus 10 g of salt makes 110 g of salty solution. Not 100 g — the salt is still in there. This links to the law of conservation of mass: in any change like this, mass is never lost.

Part 1Separating a solid from a liquid

Imagine a beaker of muddy water — sand mixed into water. The sand is insoluble, so it hasn't dissolved; it's just floating about. How do you get the clean water out? You filter it.

Filtration separates an insoluble solid from a liquid. You pour the mixture through a folded piece of filter paper sitting in a funnel. The filter paper is like a very fine sieve — full of tiny holes. The liquid particles are small enough to slip through the holes, but the solid bits are too big and get trapped.

Part 2What filtration can't do

This is the most important point — and the most common exam trap. Filtration cannot separate a dissolved solid from a liquid.

Think about why. When salt dissolves in water, its particles spread out and become as tiny as the water particles themselves. They are way smaller than the holes in the filter paper. So they sail straight through with the water — nothing gets caught.

So if you filter salty water, you get… salty water. To get a dissolved solid back, you need evaporation or distillation, which are the next two topics. Filtration is only for solids that haven't dissolved.

Part 1Getting a dissolved solid back

Filtration failed us with salty water — the salt passed straight through. So how do you get a dissolved solid back out of a solution? You get rid of the liquid instead.

If you evaporate the solvent — heat the solution so the water turns to vapour and escapes — the dissolved solid is left behind. The salt can't evaporate, so as the water leaves, the salt has nowhere to go. It stays in the dish as a solid.

You usually do this in an evaporating basin (a wide shallow dish) heated gently. As the water boils away, the solution gets more and more concentrated, until finally only the solid remains.

Part 2Slow evaporation grows crystals

There's a difference between rushing it and taking your time. If you boil the water off fast, you get a fine powder. If you let the water evaporate slowly — leaving the basin somewhere warm for days rather than boiling it — the solid forms neat, regular crystals. This slow process is called crystallisation.

Slow is better because the particles have time to line up into an orderly pattern, building proper crystals. Rush it and they get locked in place too quickly to form nice shapes.

Part 1Keeping the liquid, not the solid

Evaporation gets you the dissolved solid back — but the water is lost into the air. What if it's the pure water you want, and the salt you want to throw away? For that you use distillation.

Distillation separates the solvent (e.g. pure water) from a solution. The trick: instead of letting the water vapour escape into the room, you catch it and turn it back into a liquid. You boil the solution, the water evaporates and leaves the salt behind, then the vapour is cooled so it condenses back into pure liquid water — which you collect in a separate container.

So distillation = evaporation plus condensing. Evaporation throws the water away; distillation keeps it.

Part 2Separating two liquids

Distillation has a second use: separating two liquids that have different boiling points. Say you have a mixture of two liquids, one that boils at 78 °C and one that boils at 100 °C. As you heat the mixture, the liquid with the lower boiling point evaporates first. Its vapour travels into the condenser, cools back to liquid, and is collected — separate from the other liquid, which is still in the flask.

It's the same idea: the part that boils off first gets condensed and caught. Whether you're separating a solvent from a solution or two liquids from each other, distillation works by boiling something off and then condensing it back.

Part 1Splitting up the colours in an ink

A black felt-tip pen looks like one colour. But black ink is often a mixture of several different coloured dyes mixed together. Chromatography is the technique that separates a mixture of soluble coloured substances — like the dyes in an ink — so you can see them one by one.

It works because the different dyes travel different distances up a piece of paper. Put a spot of ink near the bottom of the paper and dip the very bottom edge into water (or another solvent). The water soaks upwards through the paper, carrying the dyes with it. Some dyes get carried a long way; others lag behind. By the time the water reaches the top, the dyes have spread out into separate marks.

Part 2The practical & reading the result

To run it properly, a couple of rules matter:

Draw the start line in pencil, not pen. If you used pen, the line itself would be an ink that separates and ruins your result — and pencil (graphite) is insoluble, so it won't move. Put the ink spots above the level of the solvent, otherwise the dye would just wash off into the water instead of travelling up the paper.

Reading the chromatogram: count the spots — that tells you how many different dyes were in the ink. A pure substance gives just one spot (it can't be separated further). A dye that travels further up the paper is more strongly carried by the solvent. You can also compare an unknown ink against known dyes: if a spot lines up at the same height, it's probably the same dye.