This month you will be constructing a paper boat to be propelled by a drop of soap. But soap isn’t a type of fuel, you say, how can it do this?
I say, through the science of surface tension. More on the explanation after the experiment.
A flat tray about one inch thick (a cookie sheet or a cake pan works), water, an index card, liquid or bar soap, scissors.
1. Fill the tray up with water (not too full, it is hard to move it without spilling it if full… in fact, do this near a sink!)
2. Cut an index card into the shape seen below
3. Place the paper “boat” in the water at one end
4. Put a small drop of liquid soap at the small triangle cut of the boat
5. Watch the action!
6. Alternative: you can touch a corner of the bar soap into the small end of the triangle as well.
You should have seen the paper boat scoot across the surface of the water, much like it was afraid of what the soap was doing to the surface. It’s true. The paper boat is being held on the surface of the water, and that surface is weakening. Here’s what I mean:
Water has a strong surface to it, called “surface tension.” The reason behind this is due to some complicated chemical explanations of the water molecule. We’ll keep it simpler and just say that water sticks to itself pretty well, especially at the surface. Some things we see because of this are leaves floating on the surface of the water, bugs skipping across the top of it, even the fact that water forms “drops” on things.
Soap weakens this surface tension. So things that were floating on the surface have a harder time being held up. In this instance, the paper boat scoots away from the weaker part of the water to the stronger, more held together part.
Will the shape of the boat affect things? Will it make it go faster, slower, sideways…? What about the material of the boat? You can have races with different boats, types of soaps…don’t just do one thing. Be a scientist and try out different things!
I hope you enjoyed this simple experiment. If you have more questions about this, or need tips about science fair ideas around this topic (or others), contact me.
Steve Davala is a middle school science teacher who likes to write both nonfiction experiment pieces as well as fiction fantasy adventures. He’s got two kids of his own, who both like science (even if what they really like doing best is mixing baking soda and vinegar.)