Use the difference

About this video

A box of steel pins falls into the sand on a beach. Picking them out one by one would take all afternoon, but a magnet lifts them out in a second and leaves the sand behind. That is the whole trick of separating a mixture: the parts of a mixture keep their own properties, so find one property where they are different, and use it.

A sieve separates by size. Small bits, like grains of sand, fall through its holes; big bits, like pebbles, stay on top. A colander for pasta is a sieve too. A magnet separates things made of iron or steel from everything else.

A filter separates bits of a solid from a liquid. It is a sheet with holes too tiny to see, like a coffee filter or a paper towel. Pour muddy water through one in a funnel and clear water drips out at the bottom, while the mud stays on the paper. But a filter can't take salt out of sea water. Salt dissolves: its particles spread out between the water particles and are just as tiny, so they go straight through the filter with the water.

To get dissolved salt back, take the water away instead. Leave salty water in a shallow dish in a warm, sunny place. Over a few days the water evaporates, but the salt does not, and white salt crystals are left in the dish. In hot places a lot of sea salt is made this way, in wide, shallow ponds by the sea.

Transcript

1. 0:00Steel pins lost in sand. Picking them out would take all afternoon. A magnet takes a second.
2. 0:09A sieve uses size. Small grains fall through the holes. Big pebbles stay on top.
3. 0:17A filter keeps the mud and lets the water through. But salt dissolved in the water goes straight through with it.
4. 0:29So take the water away instead. In the sun it evaporates, and the salt is left behind.
5. 0:39Find where the parts differ, and use it.