here is my proposed experimental

count how many drops of water a penny holds on the heads side

repeat with soap water solution

repeat 20 times

plot

assume that the drops are the same size

is this a good experiment?

looking for... surface tension?

That depends on what your trying to do

Can you assume the drops are the same size if the surface tension is different?

the surface tension

and yes that is assuming that the drops are all the same size

and I already know the answer but my biology class wants an experiment designed

But IIRC the drop size from a pipette is governed by the outside diameter of the tip and the surface tension of the liquid - and I think therein lies the reason for Mr Skeptic's question. If you are measuring the liquid with an accurate volume measure or doing the whole experiment on an accurate scale then fine. But you need to get rid of surface tension as part of your known variable measurement if it also makes up part of your question

do you think the drops will be the same size for soap solution as for pure water?

 

what happens if you vary the concentration of soap in the solution?

If you think about it, what you are building is a big drop on the penny held there by surface tension, out of little drops who's size depends somewhat on surface tension.

 

I think you should put the water on via a pipette or a syringe, so that you measure the volume not number of drops.

 

Alternately, put a little soap on the other penny, and add pure water to both.

Make sure each time you repeat the experiment you use the same amount of soap and the same type of soap. Also the type and amount of soap can influence the results significantly.