Methods of Separation in Everyday Life · Lesson 2 of 7
Sorting Solids by Size and Magnetism
“Compare sieves and magnets as ways to sort solid mixtures by different properties.”
• Explain how a sieve separates solid particles of different sizes. • Predict how changing sieve-hole size changes what passes through. • Use magnetic attraction to separate iron from non-magnetic solids. • Choose between sieving, magnetic separation, and handpicking for a dry mixture.
A sieve is a size gate
Before kneading flour, a cook may need to remove larger bran pieces or tiny stones. A sieve has many openings. Flour particles small enough to pass through collect below it, while larger particles are left above. This is a way to separate two solids even when there are too many pieces to pick individually. The method depends on a useful difference in particle size, not on the colour of the mixture.
Separating solids of different particle sizes by passing a mixture through a screen with openings: smaller pieces pass and larger pieces are retained.
The openings need a suitable size. If they are so large that both components pass through, no useful separation occurs. If too small for either component, both remain above. Construction workers use a suitably sized screen to separate sand from pebbles and stones. The holes need not be identical in every imaginable sieve, but reasonably uniform openings make a chosen size cut clearer.
Problem
A mixture contains coarse pebbles and fine sand. What should a useful sieve allow?
- 1.Its openings should be wide enough for the fine sand particles to pass.
- 2.They should be small enough to hold back the larger pebbles.
- 3.Collect the sand below and the pebbles above; check whether their actual sizes are different enough for the chosen sieve.
A magnet uses a different property
Now imagine iron nails falling into sawdust. Both are solid, and some bits of wood might have sizes similar to the nails. A magnet offers another difference: iron nails are attracted, while ordinary sawdust is not. Moving a magnet over the mixture collects the nails without handpicking each one. This is magnetic separation. The method works only when the wanted components differ in magnetic attraction.
Using a magnet to separate magnetic material, such as iron, from material that is not attracted to the magnet.
Iron-containing scrap can be lifted from some mixed waste by magnets fitted to cranes, allowing the metal to be collected for recycling. A small classroom magnet can demonstrate the same principle with nails and sawdust. Keep small iron pieces away from young children and handle sharp nails carefully. A magnet cannot pull ordinary sand or dissolved salt out of a mixture merely because they are nearby.
| Mixture | Property used | Suitable method | What is collected |
|---|---|---|---|
| Flour with larger bran pieces | Particle size | Sieving | Fine flour passes; larger pieces remain |
| Sand with larger pebbles | Particle size | Sieving | Sand passes; pebbles remain |
| Iron nails with sawdust | Magnetic attraction | Magnetic separation | Nails attach to the magnet |
| A few conspicuous stones with pulses | Visible size, shape, or colour | Handpicking | Individual stones are removed |
Sieving sorts pieces already present as solid particles. It cannot separate dissolved salt from water, and a flour-and-water mixture is not the solid-solid size comparison in this example.
Quiz
What property does sieving chiefly use?
What happens if sieve holes are larger than both solid components?
Which method best separates iron nails from sawdust?
Which example is a use of sieving?
Why is a magnet helpful in iron-scrap recycling?
Practice Problems
- Explain what should pass through and remain on a sieve used for flour and larger bran particles.
- Predict why a sieve with holes wider than both types of particles fails.
- Compare the property used for nails in sawdust with that used for pebbles in sand.
- Draw a labelled sieve showing fine particles below and coarse particles above.
- Explain why a magnet is more convenient than handpicking many nails from sawdust.
- Choose a method for each: a few stones in lentils, coarse sand and pebbles, iron powder and sawdust. Give your reason.
Key Takeaways
• Sieving separates solid particles when a useful size difference matches the openings. • Magnetic separation works when one component is attracted to a magnet and another is not. • Check the property of the mixture before selecting a method.