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Eureka! Soap Boats

Note:  This is the part of a series of posts about science activities to be done in the bathroom – the lav-oratory if you will.

I sat down today to create a new business card since it’s been over 5 years since I’ve had one and a LOT has changed. But I don’t want a boring old business card – I want a card that is also a science experiment! So this is what I came up with



You can use any business card, piece of cardboard or even a flat piece of Styrofoam (like from the top of an egg carton) to do this, not just mine, so if you won’t be running into me anytime soon just make your own!

The basic shape is a simple house with a triangle top and rectangle bottom, but feel free to mix it up and get creative.

The fuel for your boat is soap. Any old liquid soap will work. Just put a small dab on the bottom of the rectangle end of your boat. You can smear it on with your finger or use a toothpick or cotton swab.

Then just gently place your boat in the bathtub or sink and watch it zoom. (Don’t you just love the little fishies on the bottom of my tub? Ignore the soap scum please)

What happens?

The water in the bowl sticks together, especially on the top of the water. This stickiness is called surface tension and forms a skin or film on top of the water that is hard to break. This is one reason your boat floats on top of the water instead of sinking to the bottom.

The soap on the back of the boat has a much smaller surface tension than the water. The surface tension skin on top of the water pulls away from the soap like a balloon popping. This sends your boat zooming.

The fancy name for this is the Marangoni affect. It was first observed in glasses of wine – called “tears of wine.” There’s a good description here in case you want to do some science the next time you (or your parents) pour a glass of wine.

What if?

What if you cut your boat in a different shape or size? How big a boat can you power with the dish soap? Do other liquids besides soap break the surface tension skin of water and send your boat zooming? Try oil or rubbing alcohol.