Mix it up! The chemistry of mixtures and separating

by Rosie Coates

Mixtures are all around us, from a glass of milk, to the ink in a pen, to a bucket of sandy water at the beach. Often we’re happy for them to stay that way, but sometimes we need to separate them and get hold of one of the parts in a pure form.  This blogpost describes some of the chemistry used to separate mixtures, how low-tech ideas are helping recycle hi-tech waste, and activities to try in lessons for Key Stage 3 and 4.

bucketand spade on beach_AttributionRoss

Sand, Salts, Water- Seaside mixtures (Photo: Ross CC-BY-SA)

Take a bucket of seawater …

To separate two substances there needs to be a difference in one of their properties.  Salt is soluble in water and sand isn’t. To separate the sand from our bucket of sea water, we use filtration. Water boils at around 100 degrees celsius, salt doesn’t.  To separate the water and salt from the remains of our bucket we use distillation (evaporating the water and then condensing it into another container).  If there’s any oil in the water, we can take advantage of the density difference to separate the oil and water, using a separating funnel. This leaves us with sand, salt, oil and water good enough to drink; handy to know if you find yourself on a desert island!

You can find lots of activities on different topics on The Royal Society of Chemistry’s Learn-Chemistry pages, including this one about mixtures and separating.

It’s not just in the lab that separating is useful: industries from mining to recycling rely on different separating techniques.

Recycling- messy mixtures

Recycling relies on being able to sort different materials out so that they can be reformed into new objects.  You might do some of this sorting in your home, perhaps you have to separate glass from plastic in your recycling collection.

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Separate the plastic from the glass bottles? (Photo: CC-BY-SA)

As part of the recycling process different techniques can be used.  Magnetic materials such as steel can be separated using magnets, different plastics may be separated based on their density. This is often done using remarkably low-tech equipment.

Try it yourself

You can try separating different plastics yourself using density.

1. Chop up different plastics and pop them in a small jug of water.  Some will float and some will sink.

2. Scoop out the floating pieces.

3. Now add some salt to the water until some of the pieces which sank at first start to float.

4. Scoop them out.

5. Keep going until you have different piles of plastic sorted by density.

Adding salt to the water increases the density of the water, meaning that denser plastics can float in it.

From centuries old mining to circuit chips

David Wright explaining shaking table

David Wright, Chief Assayer at Geevor mine showing a family group the shaking tables (Photo: Geevor Learning- All Rights Reserved)

From the 18th Century Cornwall was famous for its tin mines.  Tin was found along with copper, gold and lots of other minerals.  One way of sorting the different materials was by using shaking tables or jigging.  Tilted tables (see photo above) are shaken and water is used to separate the material into the more dense at the bottom of the table and the less dense at the top.  Geevor mine didn’t just use them for separating materials from the mines though. In 1990 the tables were used to help in recycling printed circuit boards.  They were shredded and the shaking tables used to recover 95% of the copper. Pretty impressive stuff! The process works so long as one material is at least three times as dense as the other.

RetiredCPUs_OndrejMartinMach

Retired CPUs (Photo: Ondrej Martin Mach CC-BY-SA)

Geevor closed as a mine in 1991 but it is still very much alive and well as a museum and learning centre.  The team there have developed simple demonstrations of froth flotation, heavy media and magnetic separation, so that visitors can themselves discover how these separating techniques worked.  They are a veritable playground for practical activities so if you get the chance (perhaps you’re visiting Cornwall during the summer) or are based in the area they are well worth a visit, or check out their website for more fascinating history and science of mining.

School group panning

A school group using panning to separate materials at Geevor mine (Photo: Geevor Learning- All Rights Reserved)

Whether in the chemistry lab identifying and purifying new drugs or reprocessing and reusing materials from hi-tech industries, understanding and being able to use separating techniques is invaluable to our industries and environment, and as you can see from the children above, it can be very engrossing!

Curriculum Links

England

Pure and Impure substance

  • Mixtures
  • Simple techniques for separating mixtures

Wales

The Sustainable Earth

  • The physical and chemical properties of some elements, compounds and mixtures and how mixtures can be separated by simple techniques

Scotland

Materials- Properties and Uses of Substances

  • I can differentiate between pure substances and mixtures in common use and can select appropriate physical methods for separating mixtures into their components.

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Northern Ireland

Chemical and Material Behaviour

  • Elements, compounds and mixtures

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Posted in Chemistry, Engineering, Exploring Science, Physics