Friday, September 25, 2015

How many drops of water fit on a penny? (A Surface Tension Experiment)

My 10-year-old wanted nothing to do with this activity. How's that for honesty, eh? But he grudgingly saddled up to the stool at our counter top when a spark of curiosity ignited at the sight of a penny, eye dropper, and glass of water.

This simple experiment elicited a, "WHOA! That's cool," from a boy who was too impressed to remember he was supposed to be cranky. 

I love when that happens.

To start I asked my son how many drops of water he thought will fit on a penny.


"Well, let's see."

And we did. He filled the dropper with water and one by one, added drops of water from one inch above the penny. 

At 21 drops or so, a drop finally overflowed off the top of the penny. My son had to pick his chin up off the floor.

The water bowed out from the penny top like a mushroom top. How did it stay on top so long, without spilling?

The answer is surface tension. The tiny molecules in the water are attracted to each other. This attraction is what keeps the water droplets together. 

Now we added some liquid dish soap to the water and tried the experiment again. Would the soapy water droplets make a difference?

The answer is yes. The soap decreases the surface tension. The attraction of the molecules to each other is decreased and fewer droplets will be suspended on the penny as a result. 

This experiment came from Scientific American.

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