KS3 · Chemistry

Separating an insoluble solid from a liquid (filtration)

A tea bag keeps the leaves in and lets the tea through — so how does a bag full of holes know the difference?

Chemistry · Separating mixtures

Same problem, four different answers

Pick what the solid is like. Watch which method it points to.

The mixture → What the solid is like → The method

4 ways to separate.

On An insoluble solid mixed into a liquid. 4 branches to choose from.

You have an insoluble solid mixed into a liquid and you want it out. Which method? Look at the solid, not the liquid.

Watch out: Easy trap: reaching for a filter every time. The solid's properties choose the method, so ask what the solid is like first.

Predict, then check

Think about the tea bag in your kitchen before we meet filter paper.

A tea bag keeps the leaves inside but lets the tea through. Which rule describes what every filter does?

The filtration set-up

The filtration set-up
Conical flaskFiltrateFilter funnelFilter paperResidue

Showing 3 layers: Flask and filtrate, Filter funnel, Filter paper and residue

Layers

Explore

Tap a part to see its job. Then peel off the filter paper and see what the funnel can do on its own.

Peel this layer off and only the funnel is left. The paper is where the separation takes place.

Tap a part to see its job. Then peel off the filter paper and see what the funnel can do on its own.

Chemistry · Filtration

Which of these sounds like you?

A mixture of a liquid, an insoluble solid and a dissolved substance is poured into filter paper held in a funnel.

Which idea is closest to what you think happens?
How sure are you?

Chemistry · Filtration method

Put the method in order

the order you would do them in

1 · First step5 · Last step

  1. Pour the mixture gradually into the paper, keeping the level just below the top

  2. Fold the filter paper in half, then in half again

  3. Swirl the mixture

  4. Put the pointed end of the cone into the funnel, with a drop of water

  5. Open the paper out into a cone

Watch out: Do not poke the solid in the paper — it could rip the paper.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Choose the right method for an insoluble solid, then see how a filter does its job.

What you need to know

  • An insoluble solid will not dissolve, so it can be separated from the liquid it is mixed with.
  • The solid's properties decide the method: by hand, with a magnet, by decanting, or by filtration.
  • Filter paper has tiny holes called pores, smaller than the insoluble particles but bigger than dissolved substances.
  • The residue stays in the filter paper. The filtrate is the liquid that passes through into the flask.

The big picture

Filtration separates an insoluble solid from a liquid by passing the mixture through a filter. The solid's properties choose the method: pick big pieces out by hand, use a magnet for steel, iron, nickel and cobalt, decant a solid that sinks, and filter when the particles are tiny. Filter paper has pores smaller than the insoluble particles but bigger than dissolved substances, so the solid stays behind as the residue and the liquid, with anything dissolved in it, passes through as the filtrate.

Key points

1Big insoluble pieces (such as marbles) can be picked out by hand.
2Magnets attract steel, iron, nickel and cobalt. Most metals, including copper, are not magnetic.
3Decanting pours off the top layer, so it suits a solid that sinks, but some liquid is left behind and the separation is not complete.
4Tiny particles (such as flour, pepper or soil) are too small to pick out and hard to decant fully, so the mixture is filtered.
5The filter paper does the separating. The funnel supports the paper and the conical flask supports the funnel and collects the filtrate.
6Dissolved substances are smaller than the pores, so they pass through with the liquid.

Worked example

Problem

A pupil stirs some soil into a beaker of water. Choose a method to separate the soil from the water, say what decides it, and name the residue and the filtrate.

⚠ Watch out

Reaching for the filter every time. Filtration is for tiny particles. Big pieces come out by hand, steel and iron with a magnet, and a solid that sinks can be decanted — though decanting leaves some liquid behind.

🧠

Memory hook

SINC for the magnetic metals: Steel, Iron, Nickel, Cobalt. And for what comes out of a filter: Residue Remains in the paper, Filtrate Flows through into the flask.

✓

Check yourself

Cover the page. Flour has been stirred into water. Which method would you use, which property of the flour decides it, and what are the residue and the filtrate at the end?

Flashcards

(12)
What is filtration?
Separating an insoluble substance from a liquid by passing the mixture through a filter.
What does insoluble mean?
A substance that will not dissolve in a particular solvent (for example iron wool or sand in water).
How do you separate large insoluble solids, such as marbles, from a liquid?
Pick them out by hand.
Which metals are magnetic? (Hint: SINC)
Steel, Iron, Nickel and Cobalt. Most other metals, including copper, are not magnetic.
What is decanting, and what is its limit?
Pouring off the top layer of a mixture in which the solid has sunk. It leaves some liquid behind, so the separation is not complete.
Which method do you use for tiny insoluble particles such as flour, pepper or soil?
Filtration. They are too small to pick out and hard to decant fully.
Name some everyday filters.
A strainer, a colander, a tea bag and a coffee filter. Their small holes let the liquid with dissolved substances through and stop the larger insoluble solid.
Why does filter paper stop the insoluble solid?
Its tiny holes (pores) are smaller than the insoluble particles, so the particles are stopped.
What happens to dissolved substances in filtration?
They are smaller than the pores, so they pass through with the liquid. They are not collected in the filter paper.
What does the filter funnel do?
It supports the filter paper and sits in the top of the conical flask. The funnel is not the filter.
What are the residue and the filtrate?
The residue is the insoluble solid left in the filter paper. The filtrate is the liquid, with any dissolved substances, that passes through into the flask.
What does the conical flask do?
It supports the funnel on its narrow neck and collects the liquid that passes through.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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