Separation

Separating mixtures

A kitchen table with a sieve, packet of rice and a bag of sugar

In science, we know that mixtures are made up of different substances that are mixed together.

Some of these substances can be separated again because each substance keeps its own properties.

This is called a reversible change.

This means that even though things are mixed together, we canseparate them back out if we need to.

Watch: Separating a mixture

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Discover how the process of separation works.

Ways of separating substances

There are several ways we can separate substances.
Let's look at evaporating, filtering and sieving.

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Image caption,
Evaporation
By dissolving salt in water you make a solution. You can separate the salt from the water again by boiling the solution. The water will evaporate until it is all gone. The salt will be left behind.
Image caption,
Filtering
You can separate a mixture of sand and water by passing it through a piece of filter paper. The water is able to pass through the tiny gaps in the paper but the sand particles are too big and are left on the surface of the filter paper.
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Sieving
A mixture made of solid particles of different sizes, for example sand and gravel, can be separated by sieving.

Fascinating facts

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A substance being filtered into a beaker using filter paper

In the water cycle, the Sun evaporates water where it becomes clouds.
Evaporation accounts for about 90% of the moisture in the Earth's atmosphere.
Oceans, seas and other bodies of water are the source of about 90% of this evaporation, with the remaining 10% coming from other sources like plants.
On average, an evaporated drop of water spends around 10 days in the air before condensing and falling as rainfall.
A water purifier is a system that removes contaminants in water, usually through filtration.
Gold panning is a method of finding gold in sandy or rocky water, by sieving the water in a pan.
Filter paper holds solids and allows liquids to flow through it. That's why it's perfect for everyday tasks like making coffee.
We can see an everyday example of sievingwhen we cook, whether it's sieving flour for baking cakes, or draining water from pasta or rice.

Irreversible change

A piece of chemistry equipment. A glass flask with a long neck, a stopper and a rounded bottom. — Image caption,
Mixing vinegar and bicarbonate of soda together creates an irreversible change.

Mixing substances can cause an irreversible change, which is something that you cannotseparate again.

For example, when vinegar and bicarbonate of soda are mixed, the mixture changes and lots of bubbles of carbon dioxide are made.

These bubbles and the liquid mixture left behind cannot be separated back out into vinegar and bicarbonate of soda again as a chemical change has taken place.

Important words

An illustration of a girl eating a melting ice lolly at the seaside — Image caption,
When ice melts, this is a reversible change as it can be re-frozen

Evaporation – When a liquid turns into a gas slowly, at temperatures below its usual boiling point.

Filtration – A way of separating particles of a solid such as sand, from a liquid like water.

Irreversible change – A change that cannot be undone or reversed.

Mixture – A mixture of substances that can be elements, compounds, or both. It contains different substances that are not chemically joined to each other.

Reversible change – A change that can be undone or reversed.

Separate – To split two substances from an original substance.

Sieving – A way of separating solid substances using a mesh called a sieve, which allows the smallest particles to pass through, but not the largest particles.