More ways to separate mixtures

Michael needs some paper clips. They are somewhere in this drawer. Oops! When did you last tidy up, Michael? Paper cuttings, pencil shavings, marbles…!

Hand-picking paper clips will be difficult! There must be an easier way! The drawer contains a mixture of different things. They are mixed together, but they don't combine in a chemical sense. So, they all retain their original properties.

Metal paper clips have a unique property among the things in this drawer. They are attracted by magnets! Grab a strong magnet and move it over the messy drawer. It will collect all the metal paper clips! There are other methods of separating mixtures that also take advantage of the different properties of the mixture’s components.

Let’s try one! You’ll need a colouring marker pen, a glass of water, and a strip of paper towel. Take the paper towel strip and draw a line with a marker a couple of centimeters away from a shorter edge. Then, dip that edge of the strip in the water, but don’t let the water touch the ink. Hold it there for a moment.

The marker ink is a mixture of different pigments dissolved in water. When the strip is dipped in water, the water is drawn up through the fibres of the paper. As the water rises up the strip, the pigments dissolve further and are carried up the strip. Some pigments dissolve better than others, and these are carried up by water faster, than those that don’t dissolve very well. Pigments that are made of small particles also move faster, than pigments made of larger particles.

So the pigments move up the strip of paper at different speeds. This results in a pattern of different-coloured bands. This method of mixture separation is called chromatography. There are other chromatography techniques too, which can be used on gases as well as liquids. These techniques can help remove unwanted substances from mixtures, for example in the production of fuels such as biodiesel.

Or, like in our experiment, chromatography can also help analyse and identify substances in a mixture. This is commonly used when testing blood or urine samples for drugs, for instance. There is another mixture separation method often used in medical laboratories to separate components of blood. It uses a device that spins the blood rapidly — a centrifuge. A centrifuge consists of a holder — a rotor — in which tubes with the blood sample are placed.

When the centrifuge is turned on, the rotor starts to spin very fast, and centrifugal force starts acting on the blood inside the tube. This forces the densest parts of the blood, the red blood cells, to the bottom of the tube. The less dense white cells and platelets are forced to the middle. And the least dense blood, plasma, goes to the top. This method is called centrifugation.

It works very well not just for blood, but for separating any liquid mixtures containing tiny insoluble particles of substances that have different densities. Outside of medical laboratories, centrifugation is used to separate cream from milk, and even in washing machines, to separate water from your clothes at the end of the wash cycle! Magnetism, chromatography, and centrifugation are some of the many methods that help us separate mixtures in many different areas of everyday life. But when it comes to Michael’s drawer… good old fashioned tidying up would be most helpful!