Each separating method uses one property

Years 1–8Stage 1 to Stage 4Step through it

Each method takes out the one part that has the property it uses.

Separating a mixture, before any method is used. A glass beaker on the bench holds the mixture: Iron filings, sand, sawdust and salt, dry. Below it is a shelf of three small dishes for the parts once they are separated. Already separated: Nothing yet. — Iron filings, sand, sawdust and salt, all mixed together, dry.
Iron filings
Sand
Sawdust
Salt
Key Blue with a darker edge: water. Dashed grey arrows: water vapour leaving the dish. Short dark strokes: iron filings. Small round yellow grains: sand. Larger flat brown flakes: sawdust. Small white squares: salt crystals. The sun: the dish stands in a warm, sunny place. Small blue drops: water falling. Filled coral circle: what the step is about.

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How can four things that are mixed together be taken back out, one at a time?

Each separating method uses one property

Each method takes out the one part that has the property it uses.

Iron filings, sand, sawdust and salt, all mixed together, dry.

Key Blue with a darker edge: water. Dashed grey arrows: water vapour leaving the dish. Short dark strokes: iron filings. Small round yellow grains: sand. Larger flat brown flakes: sawdust. Small white squares: salt crystals. The sun: the dish stands in a warm, sunny place. Small blue drops: water falling. Filled coral circle: what the step is about.

Key Blue with a darker edge: water. Short dark strokes: iron filings. Small round yellow grains: sand. Larger flat brown flakes: sawdust. Small white squares: salt crystals. Small blue drops: water falling. Filled coral circle: what the step is about.

What do you think?

You add water to sand, sawdust and salt, and stir. Where is the salt now?
- Sunk to the bottom with the sand
- Dissolved in the water, still there
- Gone for good
Say why (optional)
Sand and salt water are poured into filter paper. What drips through?
- Pure water, with no salt
- Salt water
- Nothing at all
Say why (optional)
The wet mixture is left in the sun before skimming or filtering. What is left in the dish?
- Clean salt
- Salt dried onto the sand and sawdust
- Nothing: the salt goes off with the water
Say why (optional)

Your explanation

Why does each separating method take out only one part of the mixture?

The idea, step by step

Not to scale

2 · MagnetIron is magnetic, so the magnet pulls out only the iron filings.1 Iron filings · 2 Sand · 3 Sawdust · 4 Salt
3 · WaterIn water, sawdust floats, sand sinks and salt dissolves, still there.1 Iron filings · 2 Sand · 3 Sawdust · 4 Salt water1 Iron filings · 2 Sand · 3 Sawdust
4 · Too soonEvaporating now dries the salt onto the sand and sawdust: mixed again.1 Iron filings · 2 Sand · 3 Sawdust · 4 Salt
5 · SkimSawdust floats, so a spoon skims it off the top.1 Iron filings · 2 Sand · 3 Sawdust · 4 Salt water1 Iron filings · 2 Sand · 3 Sawdust
6 · FilterSand grains stay on the paper; salt water goes through.1 Iron filings · 2 Sand · 3 Sawdust · 4 Salt water1 Iron filings · 2 Sand · 3 Sawdust
7 · EvaporateThe water evaporates and the salt is back: four parts, four properties.1 Iron filings · 2 Sand · 3 Sawdust · 4 Salt

The methods in the order that gets all four parts back, and what each relies on

Method	What it relies on	What it takes out
Magnet	Iron sticks to a magnet; the other parts do not	Iron filings
Add water and stir	Some things float, some sink, some dissolve	Nothing yet: sawdust floats, salt dissolves
Skim with a spoon	Sawdust floats on water	Sawdust
Filter	Sand grains are too big to pass the paper; dissolved salt passes	Sand
Evaporate	Water evaporates; salt does not	Salt

Try it in the Lab

Practicals with real materials, each with its safety card.

Mixing and separating: sand, rice and paper clipsYears 1–2Bench practicalLow risk
Dissolving and getting it back: separating sand and saltYear 6Bench practicalLow risk
Design a separation: iron filings, sand, sawdust and saltYear 7Bench practicalLow risk
Separating sand and salt by dissolving, filtering and evaporatingYear 7Bench practicalLow risk
Water filter challenge: what filtering removes and what it cannotYear 7Bench practicalLow risk
Paper chromatography of the dyes on coloured sweetsYear 7Bench practicalLow risk
Winnowing and yandying: separating seed from husk and sandYear 7Bench practicalLow risk
Separating a sand, salt and water mixture and finding its percentage compositionYear 11Bench practicalLow risk
Panning for gold: separating a dense mineral from sand by densityYear 11Bench practicalLow risk

With a learner

Three questions to ask

Which part of the mixture would a magnet pick up, and why only that one?
Where does the salt go when it dissolves, and how do you get it back?
What goes wrong if the wet mixture is left to dry before it is filtered?

What to expect

Many learners say the salt has gone once it dissolves, or that filter paper will catch it. Step to Filter and to Evaporate and look for the salt.

What to try next

Try the four-part separation, or separating sand and salt, from Try it in the Lab, above, safety card first.

About this model

What is simplified

Not to scale: the grains, filings and flakes are drawn far larger than real ones, and far fewer.
The sand in the model is clean, washed sand. Some natural sands hold dark grains of magnetite, a mineral that a magnet also picks up.
Sawdust floats at first. Left in water for a long time it soaks up water and some of it sinks, so it is skimmed off soon after stirring. Skimmed sawdust is damp and holds a little salt water.
The steps are moments, not a timed run. In a warm, sunny place the water takes a day or more to evaporate; a school laboratory often heats the dish gently instead, as the Lab practicals above do.
Nothing is lost in the model. In a real separation a little of each part stays on the glassware, the filter paper and the wet sand.
Dissolved salt is not drawn: it is spread through the water in pieces far too small to see.
Other methods use other properties, such as sieving (the size of the pieces), panning (density) and chromatography (how strongly a dye clings to paper). They are in the Lab practicals above, not in this model.

Numbers and their sources

7.87 The relative density of iron: how many times as dense as water it is, so iron filings sink in water. The source gives the density of iron in grams per cubic centimetre; the density of water is one gram per cubic centimetre, so the relative density is the same number, with no unit. Source: PubChem, Iron (CID 23925): Density, citing Lide (ed.), CRC Handbook of Chemistry and Physics, 81st edition, read 26 September 2026.
2.65 The relative density of quartz, the mineral clean sand is mostly made of, so sand sinks in water. The source gives the density of alpha-quartz to three decimal places; it is rounded here to two. Source: PubChem, Silicon dioxide (CID 24261): Density (alpha-quartz), citing the IARC Monographs, read 26 September 2026.
0.55 The relative density of radiata pine, Australia's main plantation softwood, taken as the sawdust: below one, so sawdust floats on water. The source gives the density of Australian radiata pine in kilograms per cubic metre when seasoned; divided by the density of water it rounds to this value. Source: Business Queensland (Queensland Government), Radiata pine: properties, read 26 September 2026.
36 g The most salt (sodium chloride) that dissolves in a hundred grams of water at room temperature. The model's mixture holds far less salt than its water can dissolve, so all of it dissolves. Source: PubChem, Sodium chloride (CID 5234): Solubility, citing Haynes (ed.), CRC Handbook of Chemistry and Physics, 94th edition, p. 4-89, read 26 September 2026.
100 °C The boiling point of water at sea-level air pressure, to the nearest degree. Water evaporates well below it, and far below the boiling point of salt, so the water goes and the salt stays. Source: NIST Chemistry WebBook, SRD 69: Water, phase change data (Tboil), read 26 September 2026.
1465 °C The boiling point of sodium chloride, common salt. Source: PubChem, Sodium chloride (CID 5234): Boiling Point, citing the CRC Handbook of Chemistry and Physics, 94th edition, p. 4-89, read 26 September 2026.

Review

Demonstration: a simplified model. Checked against its written sources, 26 September 2026. Not reviewed by a qualified teacher.

Curriculum references

SC4-SOL-01SC4-WS-02SC4-WS-07ST1-6MW-SST2-6MW-SST3-6MW-SST1-SCI-01ST3-PQU-01AC9S7U06AC9S6U04AC9S3U04AC9S2I01

Reference, not a verified alignment.

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