Drops on Pennies Experiment

Author: Eric Drechsel

Abstract: In this experiment we dropped water beads on a penny and counted the number of drops the penny would support before spilling over. A meniscus formed (figure a) which held the pool to some critical point after which the water spilled over. Adding liquid soap to a “critical” pool (one near spilling) caused it to fail, while soap added to a sub-critical pool caused the meniscus to change shape (figure b).


Drops on pennies is a classic elementary school classroom activity for introducing experimental methods. It requires minimal apparatus, setup and breakdown and requires the students to design and enact a simple experimental protocol.

In our natural science inquiry class, we did the exercise in groups.

Parts List

  • 2 pennies
  • glass of tap water
  • pipette
  • liquid dish soap
  • toothpick
  • paper towels

Experimental protocol

  • Except for the last trial, water was dropped from the minimal height such that droplets were observed to separate from the pipette prior to making contact with the penny/pool
  • Between each drop, we waited for visible vibrations of the pool to subside
  • Between each trial, we cleaned and dried the pennies with water and paper towels

Part 1

figure a


  • Drop as many drops as possible on the penny.
  • Count the number of drops before water spills from penny.
  • Repeat for each group member.


penny 1 heads

  • trial 1: 10 drops
  • trial 2: 32
  • trial 3: 40

penny 2 tails

  • trial 1: 19 drops
  • trial 2: 36
  • trial 3: 39


  • A meniscus forms, preventing the water from flowing off the penny.
  • The first trial in each series yielded a much smaller drop count, possibly due to the penny being dry.

Part II: Test dependence on some variable

We chose our experimental variable to be the height of the dropper, and predicted that increasing the dropper height would cause the failure to happen after fewer drops since the kinetic energy of the drop would be larger, causing more disruption to the pool’s surface tension.


We only did on trial at each of 3 heights:

dropper height drops

        4cm     10 
        2cm     21
        1cm     27


Experimental data seems to support the hypothesis.

Part 3

  • Get the pool to the critical point where it is just about to break.
  • Gently add a drop of soap using a toothpick (don’t touch toothpick to soap).
  • Observe and record the result.

trial 1:

In our first trial, the drops went to bottom of the pool and didn’t seem to mix. The meniscus changed shape, becoming more flat, which we interpreted to mean that the surface tension decreased. Eventually, after 12 drops of soap, the pool broke. We decided that this behavior was probably due to our pool being subcritical.

trail 2:

  • In this trial, the pool burst on the application of the 1st drop of soap.


We believe that in trial 1, our pool was sub-critical so that the soap didn’t immediately cause it to fail, but simply reduced the surface tension so that the meniscus became more flat.

In trial 2 we were careful to get our pool as close as possible to critical. We believe it burst due to reduction in surface tension caused by the soap.


Surface tension is caused by the cohesion of electrically polar water molecules (they want to stay together). An explanation of the way soap modify the surface tension of the water is offered at http://www.exploratorium.edu/ronh/bubbles/soap.html. “Since the surface tension forces become smaller as the distance between water molecules increases, the intervening soap molecules decrease the surface tension.”