Ever see a bug skitter across the surface of a pond? Ever wonder why raindrops are the shape they are? This month’s simple science project will delve into the world of surface tension of water with a couple of simple experiments.
Surface tension is a barrier formed on the surface of water caused by something called “cohesion.” Liquids all have this force, a force that holds a material together. Some are stronger than others (liquid mercury is a highly cohesive liquid). Cohesion is what causes water to have that spherical shape when it falls from the sky, or sits on a counter in a droplet. Go ahead and put a tiny drop of water on your kitchen table and look at it from the side: you will notice a “bubble” shape. This is the water pulling itself together. These next experiments will demonstrate this concept, as well as show you how to break surface tension.
Materials for Experiment 1:
- cereal bowl
- ground black pepper
- liquid soap
- a toothpick
Procedure for Experiment 1:
- Fill a bowl with water
- Shake the black pepper onto the surface of the water (Notice how it stays afloat on the water? That is the surface tension holding the layer up)
- Take a toothpick and put the end into soap so that it coats the end
- Gently touch the surface of the water with the toothpick
- Did you see how the pepper shoots back in all directions? What must the soap be doing to the surface tension?
- Soap is something that actually breaks down the cohesive powers of water, making it have less surface tension.
- If you mix some soap with water and make a water droplet on your counter again, what do you think the shape would look like? Give it a try!
Materials for Experiment 2:
- liquid soap
- a toothpick
- a single staple from a stapler that is not folded in yet (open the stapler and pop out one staple, it should be in the right form)
Procedure for Experiment 2:
- Fill a bowl with water close to the top
- Very gently place the staple on the surface of the water so that it floats there. (This CAN be done, but you have to be very patient and careful!)
- Observe the surface of the water around the floating staple (it should look like the staple is pushing into the water, much like your head as it pushes into a pillow)
- Take the clean toothpick and touch the surface of the water (what do you think will happen?)
- Now take the toothpick and put it in soap again. Guess what you think will happen when you touch it to the surface of the water?
The soap once again breaks the surface tension of the water, causing the staple to no longer be supported.
Now, you’re probably thinking this is a cool thing that water does, but why is surface tension important? Insects wouldn’t be able to skip across the water without it, even your water skis keep you above the water due to surface tension. And by understanding how soap breaks down surface tension in water you have the ability to clean off your dirty dishes after you eat. Thanks, surface tension!
I hope you enjoyed this simple experiment and learned a little bit about how cool water can be. If you have more questions about this, or need tips about science fair ideas around this topic (or others), contact me below.
Steve Davala is a middle school science teacher who likes to write. He’s got two kids of his own and subjects them to these science activities as guinea pigs. Follow him on Twitter or email him at email@example.com.