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 and some water to do a fun surface tension experiment?
Racing lolly sticks – surface tension experiment
Water and a large container – we used our water table, but you could even do it in the bath.
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 little 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’ which we call surface tension. It is the surface tension that stops the lolly stick sinking. When we added washing up liquid we disrupted the arrangement of the water molecules which decreased the surface tension behind the stick.
Water molecules move from areas of low surface tension to high surface tension and so the stick moved.
More surface tension investigations
Can you make a little boat and see if that moves?
Ask a friend to blow waves over the surface of the water with a straw, does the surfboard still move when you add the washing up liquid?
Can you have a race with a friend? Who can get their lolly stick to the finish line first?
More Science for Kids
If you enjoyed this experiment we’ve got lots more science for kids you’ll love.
I also have a couple of books available filled with more simple science for kids.