IB Chemistry SLTopic 1 — Particulate Nature of MatterPaper 1 & 2Practical skill~9 min read
Separating Mixtures
Because the parts of a mixture aren’t chemically bonded, they each keep their own properties — and that’s exactly what lets us pull them apart. The trick is to spot which physical difference (solubility, boiling point, particle size, magnetism) you can exploit, then pick the matching technique.
📘 What you need to know
The parts of a mixture keep their individual properties, so they can be separated by physical means — no chemical reaction needed.
Every method relies on a difference in a physical property: solubility, boiling point, particle size, magnetism or density.
Filtration separates an insoluble solid from a liquid.
Crystallisation and evaporation recover a dissolved solid from its solution.
Simple distillation collects a solvent from a solution; fractional distillation separates liquids with close boiling points.
Chromatography separates dissolved substances by how far they travel with a solvent.
Choosing a method
There’s no single “separating machine” — you choose the method based on what makes the components different. Ask yourself two questions: is the thing I want soluble or insoluble? And are the parts solids, liquids or dissolved?
Always start by naming the physical property that differs. Examiners love the phrase “this works because the two substances have different…” — solubility, boiling point, and so on. Name the difference and you’ve usually bagged the mark.
Filtration — separating an insoluble solid from a liquid
Filtration is used when a solid won’t dissolve in the liquid it’s mixed with. The mixture is poured through filter paper: the liquid slips through the tiny pores, but the solid particles are too big to pass and get left behind.
🧪 How to do it
Fold a piece of filter paper into a cone and sit it in a funnel above a clean beaker.
Pour the mixture slowly into the funnel.
The liquid (filtrate) passes through into the beaker.
The solid (residue) stays trapped on the paper.
Filtration: solid particles are too large to pass through the filter paper, so the liquid runs through and the solid is left behind.
Good to know: for very fine solids that clog the paper, vacuum (Büchner) filtration pulls the liquid through faster, and centrifugation can spin down particles that are too small to filter.
Crystallisation — recovering a dissolved solid
If a solid is dissolved in a liquid, filtering won’t catch it. Instead we use crystallisation: gently heat the solution so some solvent evaporates, then let it cool. As it cools the solution becomes saturated and the solid comes out as crystals.
🧪 How to do it
Warm the solution to evaporate some of the solvent.
Test that it’s ready by dipping in a cold glass rod — crystals forming on it means the solution is saturated.
Leave the saturated solution to cool slowly.
Crystals grow as solubility falls; filter them off, then wash and dry them.
💡 Exam tip
Don’t boil to dryness — driving off all the solvent gives a powder, not clean crystals.
Sometimes you want the liquid back, not the solid. Simple distillation separates a solvent from a solution using a difference in boiling point. Heat the solution, and the liquid with the lowest boiling point boils off first, travels into a condenser where it cools back to liquid, and drips out as pure distillate.
Simple distillation: water boils off, condenses back to liquid in the cooled condenser, and is collected pure — the salt stays in the flask.
Fractional distillation — liquids with close boiling points
When you need to separate two miscible liquids whose boiling points are close (like ethanol and water), simple distillation isn’t sharp enough. Fractional distillation adds a fractionating column above the flask. As vapours rise and repeatedly condense and re-evaporate on the column, the liquid with the lower boiling point reaches the top first and is collected.
Ethanol boils at 78 °C and water at 100 °C, so you heat to about 78 °C to collect the ethanol first. In industry the very same idea separates crude oil into fractions — though that’s not something you’d do safely in a school lab.
Chromatography — separating dissolved substances
Paper chromatography separates dissolved substances (like the dyes in an ink) by how they balance between the paper and a moving solvent. Spot the mixture on a pencil line, dip the paper in solvent, and let it climb: substances that are more soluble and less strongly held by the paper travel further.
🧪 How to do it
Draw a pencil line near the bottom of the paper (pencil won’t dissolve in the solvent).
Place small spots of the mixture on the line.
Stand the paper in a shallow layer of solvent, keeping the spots above the solvent level.
Let the solvent rise by capillary action, carrying the components at different rates.
Dry the paper and look at the separated spots — the chromatogram.
Reading it: a substance that’s more soluble travels further up the paper; one that’s more strongly attracted to the paper lags behind. Different spots = different components in the mixture.
WORKED EXAMPLE
Name the best technique to separate each mixture, and give the physical property it relies on: (a) sand from water, (b) the coloured dyes in black ink, (c) iron filings from sulfur powder.
(a) Sand from water → filtrationSand is insoluble, so it’s trapped on the filter paper while water passes through.(b) Dyes in black ink → chromatographyThe dyes have different solubilities, so they travel different distances up the paper.(c) Iron from sulfur → use a magnetIron is magnetic and sulfur is not, so a magnet attracts only the iron.
⚠️ Common mix-up
Filtration vs crystallisation: filtration only catches an insoluble solid; if the solid is dissolved, you need crystallisation or evaporation.
Simple vs fractional distillation: use simple distillation for a solvent vs a dissolved solid; use fractional distillation for two liquids with close boiling points.
Up next: Changes of State — a closer look at how particles rearrange (and how energy flows) when a substance melts, boils, freezes or condenses.
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