How Much Water Fits On a Penny?

Materials:

• penny
• eyedropper or pipette (if you don't have one this article explains how to make one)
• water
• other liquids
• soap

Make a chart like this (or feel free to copy this one and print it):

Liquid	Predicted # of drops	Actual # of drops
1.
2.
3.
4.
5.
6.
7.

My supplies

For each liquid, you are going to use your eyedropper to carefully drop the liquid onto the penny until it spills over. Record the number of drops that fit. Try and make a prediction before you do it each time.

You can use any and all liquids you want (assuming they are non-toxic obviously). However, make sure you try this with both normal tap water and soapy water. And be sure to wash and dry your penny between each new liquid (and rinse off your dropping device as well).

What's Happening?

The atoms/molecules in a substance are all attracted to each other through intermolecular forces (IMFs). This can be because the molecules are polar (meaning one side is slightly more positive than the other) such as with water. Or it can be because the molecules are big enough that the electrons moving around can induce positive and negative poles. Whatever the reason, when the IMF is between molecules in the same substance, it is called cohesion. So all the molecules on the inside of the liquid are attracted to all the molecules around them. However the molecules on the surface of the liquid don't have any molecules above them to be attracted to, so they are more attracted to the molecules on the inside. This forms a "film" on the surface and keeps the liquid together in a drop.

Since water is polar, and therefore has high IMFs, its surface tension is relatively high. So more water should be able to fit on the penny before the surface film "breaks" and the water spills everywhere.

Image courtesy of factfixx.com

Here's my data chart:

Liquid	# of drops
1.       Water	24
2.       Water	26
3.       Soapy water	23
4.       Milk	22
5.       Salt water	14

Huh... not what I expected at all! Except for the salt water, the number of drops is pretty similar in each. So either the explanation I just gave was wrong, or something happened in the experiment that I didn't account for. Even though the number of drops was the same, I definitely felt suspicious because the blob of soapy water on the penny just didn't look as big as the one from normal water had been. I also felt like the size of the drops were smaller. So since I was using a syringe with volume measurements marked on the side, I filled the syringe up to 1 mL each time and after I had finished dropping on the penny, I continued counting drops until I had emptied the syringe.

So here's my new data table:

Liquid	# of drops	# drops in 1 mL	Ratio
1.       Water	24	23	1.0
2.       Water	26	19	1.4
3.       Soapy water	23	46	0.50
4.       Milk	22	27	0.81
5.       Salt water	14	22	0.64

Now the results make a little more sense. And it does make sense that the drops would be smaller - they have less surface tension to hold them together. I find it interesting though that none of the websites that reminded me of this activity (this isn't the first time I've done this, I actually distinctly remember doing the penny water drops experiment in 3rd grade) mentioned this fact. It might have just been the syringe I used. If anyone else tries this, I'd be interested in hearing how it worked out.

Learn more:
http://hyperphysics.phy-astr.gsu.edu/hbase/surten.html#c4
http://www.chem.purdue.edu/gchelp/liquids/tension.html