Investigating Separation Techniques in Mixtures
Every way of separating a mixture works by exploiting one physical property. Particle size, solubility, density, magnetism or boiling point. Learn the property and the six techniques stop being six things to memorise.
And the one worth doing in class is chromatography, because it takes a single black dot and pulls three colours out of it.
Mixture or pure substance
A mixture is two or more pure substances that are not chemically bonded. That is why it can be pulled apart again by physical means, with no chemistry involved.
A pure substance is one type of matter with a fixed set of properties. Water freezes at 0 °C, boils at 100 °C at standard pressure and has a density of 1 g·cm-3. Always, everywhere, in every sample.
How you actually tell, and you cannot do it by looking
Salt water looks exactly like water. Air looks like nothing at all. There are two real tests.
- Melting and boiling point. A pure substance has a sharp melting point and boiling point. A mixture melts and boils over a range, because its components have different values and they do not all change state at once
- Try to separate it by physical means. If it comes apart, it was a mixture
Chromatography is test two, made visible, which is exactly why this topic puts it here.
Homogeneous and heterogeneous mixtures
| What it looks like | Phases | Examples | |
|---|---|---|---|
| Homogeneous | The same all the way through. You cannot pick out the parts | One | Salt water, black coffee, air, steel, brass |
| Heterogeneous | Non-uniform, with visible boundaries between the parts | Two or more | Sand in water, oil and water, peanuts and raisins, stones in concrete, dust in air |
A homogeneous mixture is a solution, and a solution does not have to be a liquid. Air is a homogeneous mixture of gases. Steel is a homogeneous mixture of iron, carbon and other metals in the solid phase. Both are solutions.
The sugar example, which is the one that teaches it
Stir sugar into water and it dissolves. One phase, looks the same throughout: homogeneous.
Keep adding sugar past the point where it can dissolve and crystals settle on the bottom. Now there are two phases, a liquid and a solid, each with its own properties: heterogeneous.
Same two substances. The classification changed because the amounts changed.
Which property does the separating
This is the table to know. Every technique below is one physical property being exploited.
| Property | Technique | Works because |
|---|---|---|
| Particle size | Filtration, sieving | Large particles cannot pass through the holes |
| Solubility | Dissolving then filtering, chromatography | Some things dissolve and others do not, or dissolve at different rates |
| Density | Decantation, settling | Denser material sinks |
| Magnetism | Magnetic separation | Only some materials are attracted to a magnet |
| Boiling point | Evaporation, distillation | Components boil at different temperatures |
Evaporation and distillation both use boiling point, and the difference is what you keep. Evaporation keeps the solid left behind and lets the liquid go. Distillation collects the liquid and condenses it back.
Where this fits in the curriculum
| Subject | Physical Sciences |
|---|---|
| Grade | 10 |
| Term | 1 |
| Topic | Matter and materials, revision of matter and classification |
| Status | Class activities. Not a formal assessment |
| Marks | 40 on our worksheet. None is prescribed |
The practical, part one: filtration and evaporation
Make a mixture of salt, sand and water, and get both solids back separately.
Apparatus
| Item | Qty |
|---|---|
| Beakers, 100 ml and 250 ml | 6 |
| Glass funnels | 2 |
| Filter paper | 1 pk |
| Evaporating basins, porcelain | 2 |
| Stirring rods, test tubes and rack, spatula | |
| Bar magnet and goggles |
You supply the salt, sand, oil and iodine, all of which are kitchen or store-cupboard items.
Method
- Two spatulas of sand and two of salt into 100 ml of water. Stir well
- Ask what kind of mixture it is before separating anything. Heterogeneous, because the sand is visible
- Filter it. Fold the paper into a cone, sit it in the funnel, wet it so it clings, and pour slowly down a stirring rod
- The sand stays on the paper. The salt water passes through
- Heat the filtrate gently in an evaporating basin until it is nearly dry
- Salt crystals are left behind
The stirring rod is not optional. Pouring straight from a beaker splashes over the rim of the paper, sand gets through, and the filtrate comes out cloudy. It looks like the filtration failed when in fact the pouring did.
The practical, part two: paper chromatography
This is the one worth building the lesson around, and it runs itself once set up.
- Cut filter paper into strips about 1 cm wide
- Draw a pencil line about 2 cm from one end. Pencil, never pen
- Label each strip in pencil with the colour it will carry
- Put the colour on the pencil mark. A dot from a felt-tip, or dip a coloured sweet in water and rub it on
- Hang the strips over the edge of a beaker so the marked end reaches the bottom
- Pour water in carefully, to just BELOW the pencil line
- Leave it a few hours. Then record the colours and how far each has travelled
Step 6 is where nearly every class goes wrong
The water must start below the pencil line.
If the water covers the spot, the dye dissolves straight off the paper into the beaker. The strip comes out blank, the water goes coloured, and a run that takes several hours has been wasted.
It is the most common failure in this practical by a wide margin, and it is entirely preventable with one sentence before they start.
What you should see
| Colour tested | Usually separates into |
|---|---|
| Black felt-tip | Blue, red and yellow. The best result in the practical |
| Brown or purple sweets | Red plus blue, or red plus yellow |
| Green | Blue and yellow |
| Orange | Red and yellow |
| Red, or blue | Often stays one band. A single dye |
Make sure at least one group runs black. Most black inks are three or four dyes and the strip comes out as a clean stack of bands.
A colour that does not separate is a result, not a failure. It suggests a single pure dye, which is precisely the pure-substance-versus-mixture point the whole topic is making. Do not let a group with a single red band think theirs went wrong.
Why the colours end up in different places
The dyes have different solubilities in water. The more soluble a dye is, the further the water carries it up the paper before it settles. Less soluble dyes stay near the pencil line.
That is the property being exploited, and it is the answer to "why does this work" that earns the marks.
If it does not work
| What you see | What caused it |
|---|---|
| The strip is blank and the water is coloured | The water started above the pencil line. The commonest failure by far |
| The colour has not moved at all | The sweet coating was matt or powdery. Use the hard-shelled kind |
| The labels dissolved and ran up the paper | They were written in pen. Pencil only, and see below |
| The solvent front is crooked | The paper is creased, or the strip is touching the side of the beaker |
| Nothing has happened after twenty minutes | Normal. It takes hours. Set it up and walk away |
| The filtrate is cloudy | Poured too fast, or over the rim of the paper. Use a stirring rod |
| The evaporating basin cracked | Heated all the way to dryness. Take it off while liquid remains |
| No salt crystals appear | Not enough salt dissolved, or the filtrate was not heated long enough |
The failure worth keeping
A group that labels their strips in pen will watch their own labels dissolve and climb the paper alongside the dye.
It is a perfect, self-inflicted demonstration that ink is a mixture of dyes, and it teaches the lesson far better than the instruction they ignored.
How the 40 marks are made up
| Section | Marks |
|---|---|
| Homogeneous and heterogeneous classification | 6 |
| Definitions and the purity tests | 6 |
| Filtration and evaporation | 8 |
| Chromatography results and diagram | 10 |
| Explaining why chromatography works | 5 |
| Matching the property to the technique | 3 |
| Conclusion | 2 |
No mark allocation is prescribed. The three activities in this topic are set with no marks attached, so the worksheet and this split are ours.
The mark most often dropped on the diagram is the solvent front. Learners draw the bands and forget the line the water reached, which is the reference every distance is measured from.
And in the explanation, "because they are different colours" earns nothing. "The more soluble dyes travel further up the paper" earns both marks.
If you have time
Run black ink from three different brands of pen. They separate differently, because every manufacturer blends its own. It makes the point that "black" is not one thing.
Try a leaf. Grind spinach with a little surgical spirit, spot it on and run it. The green separates into two greens and a yellow, which is chlorophyll a, chlorophyll b and the carotenoids. It connects this practical straight to Life Sciences.
Keep the best chromatograms. They dry flat, they last for years, and a wall of them is the cheapest display a science classroom will ever have.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, with the classification table, the filtration and evaporation questions, the chromatography results table and a box for the strip diagram
- Marking memorandum, with expected results, how to mark a chromatogram that did not separate, and the five places learners most often drop marks
Related practicals
- Investigating solubility, Grade 10. The property chromatography depends on
- Heating and cooling curves, Grade 10. Melting and boiling point, which is the other purity test
- What is chromatography paper. If you want the paper rather than the method
Buy this experiment
We are putting together a Separating Mixtures Kit for Grade 10 with the beakers, funnels, evaporating basins, test tubes, stirring rods, magnet, filter paper and goggles in one box, plus a printed teacher guide and the marking memo. Coming shortly.
Filter paper is the only thing that gets used up, and it is in the filter papers range. For a school running chromatography every year, proper chromatography paper gives a cleaner separation and a straighter solvent front, though ordinary filter paper does the job the method asks for.
The sweets you buy yourself. Get the hard-shelled coloured ones, because the matt and powdery coatings do not release enough dye.