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Surface Tension

Introduction
Exploring surface tension through floating paperclips and racing boats
Useful information
Kit List: 
  • bowl or tray of water
  • paperclips
  • plastic fork or paperclip sized pieces of kitchen paper
  • paper
  • scissors
  • washing up liquid and some toothpicks or a pipette
  • rubber bands or balloon (as an analogy for surface tension).

Frequency of use: 
1
Explanation
Explanation: 

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. This works well when demonstrated with psychedelic milk where you can see patterns forming with food colouring in milk due to flows driven by variations in surface tension.

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. This is related to how insects like mosquitos and water striders can stand on relatively still water.

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 by placing it on one side of another paperclip bent at 90 degrees. This leaves the paperclip floating on the surface of the water. This may require a bit of practise beforehand and if this doesn't work then try lowering it with the paperclip at the end of a plastic fork or above a small piece of kitchen paper, which will eventually sink into the water.

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.

Without the paperclip, the surface of the water in a large container is nearly flat. This is because all the water molecules are strongly attracted to other water molecules, but not to air. Therefore the water molecules try to make the surface (the interface between the water and air) as small as possible.

The paperclip pushes down on the water. This is balanced by the surface tension of the water which acts outwards in all directions on the surface. The children should be able to see that the surface of the water rises directly next to the paperclip and this gives the upward force needed to balance the weight.

'What about pieces of paper?'

Get the children to cut up some small (about 4cmX4cm) bits of paper to make the boats. 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. This also works well when it is done as a race between two people starting at the same end of the tray. You may find that the boats stop moving after a while even as more washing up liquid is added.

What is happening here?

By adding the washing up liquid, the surface tension is reduced. Thinking about rubber bands or balloon, 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. The paper boats still float as they are less dense than the (now slightly soapy) water. The boats eventually stop moving as the washing up liquid has now spread evenly across the surface, reducing the surface tension evenly across the surface.

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.

Risk Assessment
Date risk assesment last checked: 
Sat, 27/01/2018
Risk assesment checked by: 
hannah_thorne
Date risk assesment double checked: 
Sun, 04/02/2018
Risk assesment double-checked by: 
bakrill
Risk Assessment: 
DESCRIPTION Getting paperclips and pieces of paper to move in a tray of water using surfactants
RISKS
  • Using scissors to cut out boats from card/paper
  • Lots of water present
  • Surfactants can be harmful
  • Paperclips could be sharp (especially if broken)
  • ACTION TO BE TAKEN TO MINIMISE RISKS
  • Use safety scissors. Hand scissors to parents of small children.
  • Clean up spills quickly. Ensure mopping materials are available.
  • Use harmless surfactants; keep a track of where they are. Supervise the experiment well
  • Don't let children take paperclips away
  • ACTION TO BE TAKEN IN THE EVENT OF AN ACCIDENT
  • Call first aider in case of injury
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