Getting Sorted

It’s one of those common questions – how does the science you do in the classroom work in real life? Here’s how density crops up every day, as we all try to be a little greener…

You will need:

Different plastic cartons such a milk bottle, yoghurt pot, washing up liquid bottle
Scissors
A large bowl of water
A tablespoon
Salt

What to do:

Check the code on the bottom of your containers. There should be a number inside the triangular recycling symbol; for example a plastic milk bottle is code 2 or HDPE
Cut out three strips, about 1cm x 4cm, from each plastic container.
Fill the bowl with water and place the strips in it making sure they are fully immersed. Try adding washing up liquid if the strips cling to the surface.
One set of strips will float to the surface immediately. Remove these strips.
Now add a large tablespoon of salt to the bowl and stir it up so that it dissolves. You may need to keep adding more until another set of strips float to the surface. Remove these strips.
There should be one set of strips still sitting at the bottom of the bowl.
Congratulations! You have now successfully sorted three different plastics!

What’s going on?

Each plastic container is made of a different plastic, and has its own 'density'. Density is the measure of how heavy something is for its size, in other words its mass per unit volume.

Water has a density of 1 g/cm3, and anything with a density greater than this will sink in water; anything with a lower density will float. So if we have a plastic with a density less than 1 (e.g. the milk bottle) it will float in water.

Adding the salt to the water makes it more dense. That's why we float more easily in the sea than in a swimming pool. The yoghurt pot has a density slightly greater than 1 g/cm3, so it doesn't float in tap water but it will float in salt water. The washing up liquid bottle however has a higher density still, which means that it won't float in either tap or salt water.

This is how they sort plastic for recycling, which is why you can drop all sorts of different cartons into one enormous skip!