one cup filled to the brim with water
one cup filled with pennies
(optional) hypothesis and observation sheet with pencil
Water tension is defined by Merriam-Webster as the property of a liquid surface that causes it to act like a stretched elastic membrane. Its strength depends on the forces of attraction among the particles of the liquid itself and with the particles of the gas, solid, or liquid with which it comes in contact. Surface tension allows certain insects to stand on the surface of water and can support a razor blade placed horizontally on the liquid's surface, even though the blade may be denser than the liquid and unable to float. Surface tension results in spherical drops of liquid, as the liquid tends to minimize its surface area.
Um...that's a little wordy for my 3 and 4 year old scientists! I decided not to go into any kind of formal definition and instead point out various examples of this phenomenon and give them a name for it.
We walked around our yard and noticed drops of rain on the slide and leaves from the night before.
"Why doesn't the rain just slide off?"
"I don't know.... Look, there's some more!"
"What is it doing instead?"
"It's making bubbles."
The kids had a good time finding all the examples of surface tension that we could.
Then it was experiment time. Before going any further, let me say that this experiment is definitely more appropriate for the 2nd-5th grader. If you're trying this with younger kids, I would keep things pretty simple. And expect that at least half the point will be good practice in following directions! My kids were aching to throw a bunch of pennies in at once...and I do mean throw.
Okay, back to the nuts and bolts. Set up the experiment. Have your pennies handy (we put ours in a cup) and fill a second cup to the absolute top with water. We also used an abacus to help keep track of our counting. If your kids are older, you may find this unnecessary.
Ask your kids, "How many pennies do you think we can drop into this cup of water before the water overflows?" Write down everyone's answers on your hypothesis sheet (we just verbalized our hypothesis since my kiddos are so young...which, by the way, they loved. Thank you, Dinosaur Train).
Start dropping pennies!
Careful! Notice the water everywhere. That was due to throwing a bunch in at once and starting over. Patience.
With preschoolers, it's important that everyone gets a turn ;)
As soon as you start to see the water overflowing, stop. Write down your observations. You'll be amazed at how many pennies can go in the cup before the surface tension breaks!
This would be a good point to explain to older kids why so many pennies were able to fit (the definition behind surface tension). It's also a great time to try to think of other examples...water bugs walking across the water, small objects floating on top of the water even though they sink when placed under the water, etc.
Overall, a super easy and fun experiment!
Thanks for reading!