Do you remember our lolly stick races? They are a great way to explore surface tension, but this activity demonstrating surface tension with toothpicks is even easier!
The toothpicks start off in a triangle shape and then fly apart when a little washing-up liquid is added.
Surface tension activity with toothpicks
You’ll need
- Cocktail sticks or toothpicks
- A clean bowl
- Washing up liquid /dish soap
- A large bowl
Instructions
- Fill the bowl with water and let it settle for a few minutes.
- Carefully place the cocktail sticks on the top. Try and get them to form a triangle.
- Add a drop of washing-up liquid, and watch what happens.
Why does it work? Surface tension explained!
This activity is a very simple 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.  When the washing-up liquid is added, it disrupts the arrangement of the water molecules, decreasing the surface tension inside the triangle of toothpicks.
Water molecules move from areas of low surface tension to high surface tension, and so the sticks move.
Don’t worry if that explanation seems too much for your child. I don’t always explain the full scientific principles to my children, but watching and taking part in activities like these really does encourage children to start to question why things happen, and the concepts will start to make more sense to them as time goes on.
More surface tension experiments
Try racing lolly sticks across a bath or large container of water.
Raising Lifelong Learners has a great activity for making a paperclip float using surface tension.
Watch food colouring shoot across a tray of milk.
If you enjoyed this investigation, why not try one of my other easy science experiments!
Last Updated on September 19, 2023 by Emma Vanstone
