All mixed up: can you separate it?

Some substances are single or pure substances: pure water, gold and silver, even baking soda, are made up of just one substance. It is far more common in nature, however, to find mixtures of two or more of these pure substances and we often need to separate them. Mixtures are often relatively easy to separate into their components. The method we choose for separating substances depends on the properties of the substances involved.

Sieving

The muffin recipe says ‘add the sugar and sultanas separately’ but you have mistakenly mixed them. Now you need to separate them before you can continue making the muffins. What is the easiest and quickest way to do this?

Using a sieve will allow the smaller grains of sugar to pass through the holes and retain the sultanas.

In refining metals from their ores large screening sieves are often used to separate small chunks of ore from larger ones which require further crushing; in quarries the material is graded by screening to provide different size pieces of rock for different purposes such as road building and concrete mixing.

Decanting

Sometimes you may want to separate an insoluble solid substance which is mixed with a liquid, for example a bottle of red wine which contains sediment.

Decanting is a method you can use to roughly separate the liquid from the solid. To do this, allow the solids to settle to the bottom of the bottle then gently pour the liquid off the top, trying not to shake the bottle. This will leave the sediment in the bottom of the bottle.

Gravity separation

Gold panning is a well-known example of gravity separation. A stream of water is used to move the lighter particles of sand away from the heavier gold particles.

Centrifuging

Sometimes it is not possible to separate the heavier particles in a mixture using the gravity method. A centrifuge separates a mixture by spinning. Cream is separated from milk by this method and in medicine a centrifuge is used to separate red blood cells from the blood plasma. The spin-dryer in a washing machine is a form of centrifuge and the same method is used to separate sugar crystals from liquid sugar.