Mixtures and separation · Process
Filtration
Getting sand out of water takes one piece of paper. Getting salt out of water cannot be done with any paper at all — and the reason is a matter of size.
Start here
Two beakers of water. One has sand in it, one has salt. The same filter paper is about to meet both.
Stir a spoon of sand into the first: it goes cloudy and the grains settle. Stir a spoon of salt into the second: it goes clear and stays clear. Pour each through a filter paper.
What is left in the two filter papers afterwards?
Sand in the first paper. Nothing at all in the second — the salt went straight through with the water and is still in the liquid underneath, exactly as it was. Filtration separates an insoluble solid from a liquid, and never separates anything dissolved. That one sentence is the whole lesson; the rest is how to do it without ruining it.
The solid caught in the paper is the residue. The liquid that comes through is the filtrate. Both are worth keeping — which one you actually want depends on what you are doing.
Watched first · five steps
Filtering sand out of water
Each step has a reason. Reveal them one at a time — and there is a prediction to make before you pour.
Fold the circle of filter paper into a cone.
In half, in half again, then open one side out so it makes a cone with three thicknesses on one side and one on the other.
A cone fits the funnel. A flat disc laid in the funnel leaves gaps at the edge, and the mixture runs round it.
Sit the cone in the funnel and wet it with a little distilled water.
A few drops, then press the paper gently against the glass.
Wet paper clings to the funnel. Dry paper lifts as the mixture arrives, and the mixture runs down the gap unfiltered.
Stand the funnel in a conical flask, with its tip touching the inside wall.
The stem should reach inside the flask, not hover above it.
Touching the wall stops the filtrate splashing back up into the paper, and stops drops running down the outside of the flask.
Pour the mixture down a stirring rod, keeping the level below the rim of the paper.
The rod touches the paper on the three-thickness side. Fill to about two thirds of the paper, no higher.
Poured straight from the beaker it splashes over the rim, and anything that goes over the rim has not been filtered.
Before you pour: where does the salt beaker's salt end up?
Let it drip through, then rinse the residue with a little distilled water.
Do not squeeze or poke the paper. Rinsing washes the last of the filtrate out of the sand.
Squeezing tears the paper and lets the residue through. Rinsing matters when you want a clean residue — otherwise it dries with dissolved substances in it.
Sand in the paper, water in the flask — and in the other beaker, salt water in the flask and a filter paper with nothing caught in it at all. Nothing was removed from the salt water at all.
Your turn · build the same sequence
Same five steps, shuffled. Put them in order.
Tap them in the order you would do them. Nobody is marking this — you will simply be told what happened on the bench as a result.
- Fold the circle of filter paper into a cone.
- Sit the cone in the funnel and wet it with a little distilled water.
- Stand the funnel in a conical flask, with its tip touching the inside wall.
- Pour the mixture down a stirring rod, keeping the level below the rim of the paper.
- Let it drip through, then rinse the residue with a little distilled water.
That is the order, and here is what came out.
Paper folded, seated and wet, funnel standing in the flask, mixture poured down the rod below the rim, then rinsed. Clean sand in the paper, clear filtrate in the flask, nothing spilled. Every step you did was protecting the step after it.
Why it works · and why it cannot work on salt
The holes in the paper are the whole story
Key fact
Filtration separates an insoluble solid from a liquid. The solid left in the paper is the residue; the liquid through it is the filtrate. Anything dissolved goes straight through.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“I filtered the pond water and it came out clear, so it is clean water now.”
It went in murky and came out clear. Commit before you read on.
Clear means the bits you could see are gone. Everything dissolved is still there — and so is almost everything dangerous. Bacteria are far smaller than the mud you removed and most of them pass through filter paper without noticing it. Dissolved lead, nitrate and pesticide are single particles and pass straight through.
A water treatment works does filter — and then it settles, adds chemicals to clump the fine particles, filters again through sand, and finally kills what is left with chlorine or ultraviolet light. Filtration is one step of several, and it is the step that deals with the least dangerous thing in the water.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
A sand and water mixture is filtered. What are the sand and the water now called?
Rung 2 · The one that catches people
A student cannot separate salt from water with a filter and asks for finer filter paper. What should you tell them?
Rung 3 · Explain
Sand and salt are both stirred into the same beaker of water and the mixture is filtered. Explain exactly what is in the filter paper and what is in the flask afterwards, and how you know.
Rung 4 · Take it somewhere new
A camping filter bottle claims to make river water safe to drink. Using what you know about filtration, explain what such a bottle can and cannot do, and what you would want to know before trusting it.
Key note
Filtration separates an insoluble solid from a liquid. The solid caught in the paper is the residue; the liquid that passes through is the filtrate. It works because the solid is in lumps far larger than the gaps in the paper — so anything dissolved, being single particles far smaller than those gaps, goes straight through. Clear is not the same as pure.
Going further
A filter paper is not a sieve with holes in it. It is a tangle of cellulose fibres, and a particle can be caught by hitting a fibre, by sticking to one, or by being trapped in a bend well below the surface — which is why a paper stops particles smaller than the widest gap through it, and why pouring too fast pushes them through anyway. Slow filtration is cleaner filtration, and that is not a rule about patience.
Nothing on this bench will take the salt out of sea water, because the salt is dissolved and dissolved things go where the water goes. The way round it is to move the water and leave the salt behind — which is the next lesson but one. Industry does have filters fine enough to hold back dissolved particles: reverse osmosis membranes desalinate sea water by forcing it through under enormous pressure. They are not filter paper, and the pressure is the reason they are expensive.
Before this lesson
Next in this unit
At GCSE this becomes
- Choosing and justifying a separation method for a given mixture, and required practicals where the technique itself is assessed.
Where to next
Ask Mr Badmus AI
Still not sure why no filter can hold back dissolved salt?
Filtered pond or river water is not drinking water. The filter takes out what you could see and leaves everything that could make you ill, so it is never tasted or drunk.
Lesson content © MrBadmusAI.