Skip to content

Separating

Radiation and the atom · level 1, lesson 3 of 6

A question to start

Someone has spilt a box of paper clips into a jar of sand. How could you get them all out without picking through it grain by grain?

The idea

Because the things in a mixture are still themselves, you can separate them again. You use something that is different about them.

  • Size: a sieve lets small things through and keeps big things back.
  • Magnets: a magnet pulls out iron and steel, and leaves the rest.
  • Floating: shake a mixture in water, and things that float can be skimmed off.
  • Filtering: a paper filter lets water through but keeps sand back.
a sieve: sorts by sizea magnet pulls out the clips

Worked examples

1. The paper clips. Move a magnet through the sand. The clips jump onto it; the sand stays behind.

2. Pebbles and sand. Shake them in a sieve. The sand falls through the holes, the pebbles stay on top.

3. Muddy water. Pour it through a coffee filter. Clearer water drips through, and the mud stays in the paper.

Try it

  1. Why can a mixture be separated again?
  2. How would you get iron nails out of a box of sawdust?
  3. What does a sieve sort things by?
  4. How could you get sand out of water?
Answers
  1. The things in it are still themselves, so you can use what is different about them.
  2. With a magnet.
  3. Their size.
  4. Pour it through a filter, like a coffee filter.

Common mistake

"A sieve can take salt out of sand." Salt grains and sand grains are about the same size, so a sieve cannot sort them. You need a different way, as the next lesson shows.

Where it leads