Speedboat Stick

I’ve written before about the joys of bath-time in our house here. If like me you have to do a lot of negotiating to get your children in the bath then this experiment might help with the process. It did with mine. Although it wasn’t quite as exciting as promised it did lure them into the bathroom and then they happily jumped into the bath afterwards to play with the ice lolly sticks (oh the excitement!). Anyway point is it was an easy way to get the kids in the bath and there was some science thrown in for good measure.

What you need:

  • Ice lolly stick (essential – washed and clean too. Any wooden stick from any ice lolly will do).
  • Washing up liquid (essential).
  • Bath with water in it (essential).

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What to do:

  • Run the bath as you would usually do for your kids but DON’T add anything to the water (no bubble bath etc).
  • Pop the ice lolly stick in the water at one end of the bath. It should float.
  • Put a couple of drops of washing up liquid on one end of the stick and watch what happens.
  • Get the kids in the bath and get them clean.

Tips/extra bits:

  • We have been collecting ice lolly sticks for a while so had a couple on hand. This is what the kids wanted to play with in the bath afterwards so was good to have a collection.

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The science bit

The ice lolly stick floats on the water due to surface tension. This is basically the water molecules on the surface forming a ‘skin’, which stops the stick from sinking. This occurs because the molecules on the surface are being pulled together more tightly than the molecules underneath, due to them not having water molecules above them pulling up (the molecules underneath are being pulled equally in all directions).

When the washing up liquid is added it decreases the surface tension by disrupting the water molecules. The molecules on the surface are now attracted to the washing up liquid as well as the molecules to the side and below them. Water molecules move from areas of lower surface tension to areas of high surface tension (i.e. the area of water that has been disrupted by the washing up liquid to an area of water that hasn’t). And the stick moves along with the water molecules trying to do this (ours didn’t exactly whizz but it apparently should!).

Explaining it to children

If they have ever seen pond skaters then you could say the ice lolly stick is floating on the water in the same way that pond skaters do. Explain the water like everything is made up of tiny molecules (or pieces) that you cannot see. These molecules on the top of the water make a layer that the ice lolly stick is sitting on. Adding the washing up liquid changes the molecules under the stick and they want to move to a bit of the bath that doesn’t have the washing up liquid in. This causes the stick to move along in the bath like a speedboat (hopefully).

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Experiment based on an activity from the fantastic Cool Science Tricks book by Daniel Tatarsky.

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