Overview

Showing that surface tension can support paperclips, but can be broken down by washing up liquid (surfactant) - this can be used to make 'boats' move across the water.

What can you show with it?

Paperclips lowered gently onto the surface of water can float.

Pieces of paper floating on the surface will move away rapidly when

surface tension is reduced by adding washing up liquid.

Other things to talk about

Washing up liquid breaks down the surface tension of grease on pans,

allowing it to be easily removed from the pan.

What makes an object float or sink?

Tips for demonstrating

Alternatively, I found that the water needs to be REALLY clean to float the paperclips (as do the paperclips), so it can be easier done separately in a cup that was easy to wash. This also means the paperclip can be left floating while playing with boats.

Use a generous amount of washing up liquid to reduce the surface
tension.

Basic Procedure and Explanation

'Will a paperclip float or sink when dropped in water?'

Get the children to drop a paperclip into the water from a sensible

height (>5cm) to show that paperclips usually sink. You can also

usefully talk about what makes things float or sink - ask about common

objects e.g. apples (apple bobbing at Halloween), boats, etc.

'What about if I put the paperclip in gently?'

Now lower a paperclip in gently on the end of the plastic fork, or put a

paperclip on a piece of kitchen paper on the surface (again gently). The

kitchen paper should eventually get so sodden that it sinks, but this

can take time, so using the fork may be your best bet. This leaves the

paperclip floating on the surface of the water. This may require a

bit of practise beforehand.

Now get the children to look closely at the paperclip = they should be

able to see that the paperclip is pushing down the 'skin' of the water.

So the paperclip is pushing down on the water, and the water is pushing

back up - this push is due to surface tension.

'What about pieces of paper?'

Get the children to cut up some small (about 4cmX4cm) bits of paper.

They can do this while you rinse out the tray at the beginning of the

experiment. Get them to place the pieces of paper on the surface of

the water - gently, so as not to sink the paperclip!

Using the pipette, or a toothpick dipped in washing up liquid, put some

washing up liquid into the water. Try to do this gently so that you

don't get accused of making ripples. Also, try to put the washing up

liquid between some pieces of paper so that they move apart in different

directions.

The paper pieces should move away fairly quickly, and the paperclip

should sink.

By adding the washing up liquid, the surface tension is reduced. Thinking about rubber bands, a stretched rubber band will ping away when cut (the tension in the band is reduced, but the tension at the ends remains). Reducing the surface tension in the water makes the water ping away, carrying the paper with it. The paperclip sinks because the surface tension is reduced so much that it can no longer be supported. There is still enough surface tension for the paper to continue to float.

What happens if we add soap to the floating paperclip?

You might want to remind them that the surface tension is keeping the paperclip 'floating'". Smart kids say that the paperclip would whizz. REALLY smart kids say that the paperclip would sink. You can make a big deal of the fact that they are making a prediction based on previous observations in relation to a hypothesis and then testing it, just like 'real' scientists.

The paperclip sometimes whizzed a little bit before it sank, but not much.

Get the kids to put soap in with their fingers, and have towels available for cleanup.

Some of the kids might think that you made a wave by putting your finger in the water but get them to test this by putting a soap-free finger in and actually trying to make a wave. Obviously, the boats didn't whizz.