Separating mixtures
Use a property the parts don't share
1. Separating mixtures: from zero
2. Filings in the sand
Iron filings got mixed into dry sand. The grains are about the same size.
Pick the right answer
Predict: what is the quickest way to separate them?
- A. Shake the mixture through a sieve
- B. Pass a magnet over the mixture
- C. Add water and stir until both dissolve
Explanation
A magnet attracts iron but not sand. The grains are about the same size and neither dissolves in water, so a sieve or water would not separate them.
3. Magnetic separation
To separate a mixture, use a property its parts don't share.
Magnetic separation
A magnet attracts the particles that are magnetic. The other particles stay behind.
No new substance forms. The iron is still iron, and the sand is still sand.
4. Held up by the water's surface
Surface tension
The pull between water particles at the surface. It makes the surface act like a thin stretched skin.
This skin can hold up some small, light particles. Others sink to the bottom. So the two kinds come apart.
5. Electric separation
Some particles take an electric charge more easily than others. A charged plate attracts them, and the rest fall straight down.
Match the pairs
Match each difference to the method that uses it.
- One part is attracted by a magnet
- One part stays on the water's surface
- One part takes an electric charge more easily
6. Choose the method
Pick the right answer
Two fine powders are mixed. Particles of one powder stay on the water's surface; the other powder sinks. Which method works, and why?
- A. Pass a magnet over it, because the powders differ in magnetism
- B. Sprinkle it on water: surface tension holds up one powder and the other sinks
- C. Heat it strongly to turn the sand into something new
Explanation
The difference given is how the powders behave on water: surface tension holds up particles of one powder while the other sinks, so separation by surface tension works. No new substance forms.
True or false?
After a magnet separates iron filings from sand, the filings have become a new substance.
Explanation
Separating a mixture forms no new substance: after the magnet separates them, the filings are still iron, still grey and still attracted by the magnet.
7. Summary
- Use a property the parts don't share. No new substance forms.
- Magnetic separation: a magnet attracts the magnetic particles.
- Surface tension: some small particles stay on top; others sink.
- Electric separation: a charged plate attracts some particles.
| Difference between the parts | Method |
|---|---|
| One part is attracted by a magnet | Magnetic separation |
| One part stays on the water's surface | Separation by surface tension |
| One part takes an electric charge more easily | Electric separation |
Can you explain why a sieve could not separate the iron filings from the sand in the hook?
Next: iron ore is concentrated with these same three methods.