How do you make a lolly stick race across water? Would you believe me if I told you all you needed was a drop of washing-up liquid ( dish soap ) and some water to do a fun surface tension experiment?
Racing lolly sticks – surface tension experiment
A large container – we used our water table, but a bath would work too.
Washing up liquid – dish soap
Wooden lolly stick
Fill your container with water. You only need it a couple of cm deep.
Place the lolly stick on the water. It should float.
Add a little washing-up liquid at one end of the stick. It should zoom off.
Why does this work?
This simple science investigation is a great demonstration of surface tension. When you have a container full of water, the water molecules below the surface are pulled together equally in all directions, but those on top are pulled together more tightly, as they don’t have water molecules above them. This draws them together to form a kind of ‘skin’ that we call surface tension. It is the surface tension that stops the lolly stick from sinking. When washing-up liquid is dropped onto the surface of the water it disrupts the arrangement of the water molecules, decreasing the surface tension behind the stick.
Water molecules move from areas of low surface tension to high surface tension, and so the lollystick stick moves.
More surface tension investigations
Make a small boat and investigate to see whether disrupting the surface tension of the water makes it move.
Ask a friend to blow waves over the surface of the water with a straw, investigate whether the surfboard still moves when washing up liquid is added.
Set up a lollystick race for multiple children!
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Last Updated on September 8, 2023 by Emma Vanstone