Describe everyday methods for separating mixtures, including filtering, evaporation and using a magnet, and choose the right method for a given mixture

What this dot point is asking

This dot point wants you to know simple, practical ways to separate the substances in a mixture, and to choose the right method for a given mixture. Because the parts of a mixture are not chemically joined, they can be separated using physical methods. You should be able to describe filtering, evaporation, decanting and using a magnet, and explain why each one works. The big idea is that you choose the method based on a difference between the substances, such as size, whether something dissolves, or whether something is magnetic.

The answer

Why mixtures can be separated

A mixture is made of two or more substances that are mixed together but not chemically joined. Because they are not joined, each substance keeps its own properties, and we can use a difference between them to pull them apart. The skill is spotting which difference to use.

Filtering

Filtering (also called filtration) separates an insoluble solid from a liquid. An insoluble solid is one that does not dissolve, such as sand in water.

You pour the mixture through a piece of filter paper folded into a funnel. The filter paper has tiny holes. The liquid passes through the holes, but the solid grains are too big to pass through, so they are trapped on the paper. The liquid that passes through is called the filtrate, and the solid left on the paper is the residue. Filtering is how a coffee filter keeps the grounds out of your cup.

Evaporation

Evaporation separates a dissolved solid from a liquid, for example salt from salty water. A dissolved solid cannot be filtered out because its tiny particles fit through the filter paper.

Instead, you heat the mixture gently. The liquid turns into a gas and escapes into the air, leaving the solid behind. When all the water has evaporated, the salt is left in the dish. This is how salt is made from seawater in salt farms.

Decanting

Decanting is simply pouring off a liquid carefully to leave behind a solid that has settled at the bottom. If you let muddy water stand still, the heavier mud sinks to the bottom, and you can gently pour off the clearer water on top. Decanting is quick but not as clean as filtering, because some solid may still be poured off by accident.

Using a magnet

Using a magnet separates a magnetic material from a non-magnetic one. Iron, steel, nickel and cobalt are magnetic; most other materials, including plastic, glass, copper and aluminium, are not.

You move a magnet over the mixture. The magnetic pieces are attracted and stick to the magnet, so you can lift them out, leaving the rest behind. Scrapyards use big electromagnets to lift iron and steel out of mixed scrap.

Choosing the right method

Pick the method from the difference between the substances:

• Insoluble solid in a liquid: filter it.
• Dissolved solid in a liquid: evaporate the liquid.
• Solid settled under a liquid: decant the liquid off.
• A magnetic solid mixed with a non-magnetic one: use a magnet.

Examples in context

Example 1. Making a cup of tea. When you use a tea strainer or a tea bag, you are filtering. The hot, flavoured water passes through the small holes, while the tea leaves are too big to pass and are held back. The drink in your cup is the filtrate.

Example 2. A recycling plant. At a recycling centre, mixed metal scrap is passed under a powerful electromagnet. The iron and steel are attracted and lifted away, while aluminium drink cans, which are not magnetic, are left behind to be collected separately. The whole sort is done using the difference in magnetism.

Try this

• Cue. Name the best method to separate small stones from water. The stones do not dissolve and are too big for filter paper, so filtering removes them (or decanting, by pouring the water off).

• Cue. Explain why you cannot use filtering to get sugar back from sugary water. Sugar dissolves, so its tiny particles pass straight through the filter paper; you must evaporate the water instead.

• Cue. A mixture has iron filings and sand. Describe how to separate them. Move a magnet over the mixture: the iron filings are magnetic and stick to the magnet, leaving the sand behind.