Separation Techniques — Filtering and Evaporation
In 1854, London physician John Snow traced a cholera outbreak that killed 500 people in 10 days to a single contaminated water pump — understanding how to separate particles from water was literally a matter of life and death.
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Q1 · You spill some iron filings into a bowl of salt. What is the simplest way to get the iron back out? (Hint: think about which property of iron is different from salt.)
Q2 · You have a jar of muddy creek water and you want clear water at the bottom and the mud trapped on top. What tool would you use, and why?
● Know
- The four key physical properties used to separate mixtures (particle size, boiling point, solubility, magnetism)
- The filtration procedure and what each piece of equipment does
- The evaporation procedure and how it recovers a dissolved solute
● Understand
- Why filtration works for a solid in a liquid but not for a solution
- Why evaporation recovers the solute (not the solvent)
- Why we look at property differences to pick the right method
● Can do
- Choose the right separation technique for a given mixture
- Sketch and label a filtration setup
- Apply lab safety rules to a separation experiment
- Filtration
- Evaporation
- Residue
- Filtrate
- Soluble
- Can dissolve in a liquid
- Solid trapped on the filter paper
- Boiling off the solvent to leave the solute
- Separating solid from liquid using a filter
- Clear liquid that passes through the filter
Pour sand into a bucket of water and stir — the sand sinks to the bottom while the water stays on top. Those two parts of the mixture already differ in a physical property (particle size vs dissolved), which is exactly what you need to separate them. Every separation method uses one rule: exploit a physical property difference between the parts of the mixture to pull them apart. Four common property differences:
| Property difference | Best method | Example mixture |
|---|---|---|
| Particle size | Filtration | Sand in water |
| Boiling point | Evaporation / distillation | Salt in water |
| Solubility | Dissolving + filtering | Salt + sand |
| Magnetism | Magnet | Iron filings + sulfur powder |
Important: separation methods are physical changes only. They split a mixture, they don't break a compound apart. (Splitting H₂O into H and O needs electricity — that's chemistry.)
To separate a mixture you look for a property that is different. Different particle means you can use filtration. Different point means you can use evaporation. If one part is , you can use a magnet.
Filtration separates an insoluble solid (like sand) from a liquid (like water). The filter paper has tiny pores — water molecules slip through easily, but sand grains are way too big to fit.
Equipment list: filter paper, filter funnel, conical flask (or beaker), retort stand and ring (optional, to hold the funnel), the muddy mixture, a stirring rod.
Procedure:
- Fold the filter paper into a cone and place it inside the funnel.
- Sit the funnel on top of the conical flask (or in a stand).
- Pour the muddy water down a glass stirring rod into the funnel — never directly, or you'll splash.
- Wait. Water (the filtrate) drips through into the flask. Sand (the residue) stays on the paper.
- Rinse the residue with a little clean water to wash any trapped filtrate through.
Safety: wear safety glasses (splashes), keep the funnel stable, no flames near the workbench.
Saltwater can't be filtered — the salt has dissolved into individual particles much smaller than the pores in filter paper. To get the salt back, we use evaporation.
How it works: water has a much lower boiling point (100 °C) than salt's melting point (about 800 °C). Heat the saltwater. The water turns to steam and floats away. The salt stays behind in the dish as a white crust.
Equipment: evaporating basin, tripod, gauze mat, Bunsen burner, heatproof mat, tongs, safety glasses.
Procedure:
- Pour the saltwater into the evaporating basin.
- Set the basin on a gauze on top of a tripod.
- Light the Bunsen burner. Use a low (yellow) flame to start, then a gentle blue flame.
- Heat gently. Stop when only a small amount of liquid is left — the last bit will dry on its own.
- Let it cool. Use tongs — the basin is hot.
Why not boil it dry hard? A vigorous boil will spit hot salty water onto your skin or the flame — a burn risk and a contamination risk.
Real example: the Coorong salt pans in South Australia produce table salt this way using sun and wind to evaporate seawater — no Bunsen needed.
What if a mixture has THREE parts? Use the methods one after another. Imagine a beaker of salt + sand + water. The plan:
- Filter the whole mixture. The sand (insoluble) stays on the filter paper. The salt has dissolved, so it passes through with the water as the filtrate.
- Evaporate the filtrate. The water boils off as steam. The salt stays in the evaporating basin.
That's it — three substances pulled apart by combining two physical separation techniques. The trick is to think about which physical property differs at each step.
Wrong: "Filtering saltwater will separate the salt from the water." Salt has dissolved into tiny particles much smaller than the filter pores — it passes straight through with the water.
Right: Filtration only works for INSOLUBLE solids. For a dissolved solute like salt, you need evaporation or distillation.
Wrong: "After evaporating saltwater, the salt is gone." The water is gone (as steam). The salt is still there, sitting at the bottom of the basin as white crystals.
Right: Evaporation removes the SOLVENT (water). The SOLUTE (salt) is what you keep at the end.
Wrong: "Boil saltwater as hard as possible — it'll evaporate faster." A vigorous boil spits hot saltwater out of the basin. You burn yourself and lose the salt as droplets.
Right: Heat gently. Stop the heat just before the basin runs dry so you don't crack it or burn the residue.
A scientist has a beaker of pond water that looks slightly green and has a few twigs and bits of leaf floating in it. They filter it. Predict: after filtering, will the water be perfectly clear and colourless? Why or why not? (Think about what the filter can catch and what it can't.)
How close was your prediction?
Nice — you saw that the dissolved colour particles slip through the filter.
Good — filtration only catches what is bigger than the filter pores. That's an important limit.
At the start of this lesson you were asked: Filter paper has tiny holes that let water through — but how come salt water passes straight through too? Did that puzzle you before you started?
Now write a full explanation of why filtration doesn't separate dissolved salt from water, and name the technique you'd actually need to use instead. Use the words particle size and dissolved in your answer.
Q1. Describe the filtration procedure for separating sand from water. Name the equipment, the residue and the filtrate. (3 marks)
Q2. Choose the best separation method for each mixture and give one reason: (a) iron filings + salt; (b) sand + water; (c) salt + water; (d) chalk dust + water. (4 marks)
Q3. A student claims: "If you filter saltwater long enough, eventually the salt will be trapped on the paper and you will get pure water." Evaluate this claim using your knowledge of particle size and solubility. Suggest a better method. (4 marks)
Answers
▾MCQ 1
C — Filtration works on particle size. Tiny pores in the paper let small particles through and trap large ones.
MCQ 2
B — The solid left on the filter paper is called the residue. The clear liquid in the flask below is the filtrate.
MCQ 3
D — Salt dissolves into tiny ion-sized particles much smaller than filter pores, so they slip straight through with the water.
MCQ 4
A — Filter first to catch the sand. Then evaporate the filtrate to recover the salt.
MCQ 5
C — The water is the solvent, and it leaves as steam. The salt is the solute, and it stays as solid in the basin.
Short Answer 1
Model answer: Fold a piece of filter paper into a cone, place it inside a funnel, and sit the funnel on a conical flask. Pour the muddy water down a glass stirring rod into the funnel to avoid splashes. The sand (the residue) stays on the filter paper. The clear water (the filtrate) drips into the conical flask. Wear safety glasses throughout.
Short Answer 2
Model answer: (a) Iron + salt → use a magnet (iron is magnetic, salt is not). (b) Sand + water → filtration (sand is insoluble; particle size difference). (c) Salt + water → evaporation (salt is dissolved; different boiling points). (d) Chalk dust + water → filtration (chalk is insoluble and the dust particles are large enough to be caught by filter paper).
Short Answer 3
Model answer: The student is wrong. Salt dissolves in water — it breaks into tiny dissolved particles much smaller than the pores in filter paper. No matter how long you filter, those particles slip through with the water, so the filtrate will still be salty. The student is confusing dissolved salt with insoluble grains of salt. A better method is evaporation: heat the saltwater in an evaporating basin so the water boils off as steam, leaving the salt behind. Even better, if you want pure water as well, use distillation — heat the saltwater and then condense the steam back to liquid water in a separate container.