Separating Mixtures: Sorting, Sieving, Filtering, Magnets

≈ 20 min

Because the materials in a mixture keep their own properties, you separate them by finding a property where they differ and then using a method that exploits that difference.

Method	Property it uses	Good for
Hand sorting	size, colour, shape you can see	stones out of dried beans
Sieving	particle size	sand from gravel; lumps out of flour
Filtering	one material dissolves, the other does not	mud out of river water
Using a magnet	magnetic materials	iron or steel out of anything else

Filtering deserves a closer look. Filter paper is a sheet full of holes far too small to see. Water slips through the holes; the solid particles of sand or mud are too big and stay behind on the paper. The trapped solid is called the residue and the liquid that runs through is the filtrate.

Worked example — a three-material mixture.

You are handed a beaker holding iron filings, sand and salt all mixed together. Separate all three.

Magnet. Draw a magnet over the mixture. The iron filings jump up; sand and salt do not. Iron recovered.
Add water and stir. The salt dissolves; the sand does not.
Filter. The sand stays on the filter paper as residue. The salty water runs through as filtrate.
Leave the filtrate in a warm place. When the water evaporates the salt is left behind in the dish.

Each step used a different property: magnetism, then solubility, then particle size.

What is the quickest way to separate iron filings from a mixture of iron filings and sand?

Pour the mixture through filter paper

Draw a magnet over the mixture

Add water and stir

Heat the mixture until the sand melts

The method that uses filter paper to separate an insoluble solid from a liquid is called ______.

Why can filter paper not be used to separate salt from sea water?

The salt has dissolved, so its tiny pieces pass through the holes together with the water

Salt is too heavy for the paper to hold

The paper dissolves in salty water

Salt is magnetic, so it clings to the paper instead

A learner sieves $350$ g of a stone-and-sand mixture. She collects $120$ g of stones on top of the sieve, and the rest is sand. What mass of sand passed through, in grams?