Water surface tension - pennies in a cup

Note: We got the idea for this experiment from Science Play book by Jill Frankel Hauser.

I grew up with a scientist mom, and her favorite experiment was the drops of water on a penny to analyze surface tension of water. That's a little too hard for my 3 year old to do (I remember being somewhere around 7 for those experiments). So when I found the experiment with placing pennies in a cup of water, I figured it would work for J.

Now, being a scientist myself, I had questions of my own that weren't answered by the book, like how does temperature affect surface tension of water? I decided that could be our experiment.

To complete the experiment, I filled up clear plastic glasses with different temperature water (cold water from our fridge, cold water from the sink, hot water from the sink - ours comes out really hot!) almost to the top of the glass. While the glass was on the table, I used a clean medicine dropper to fill the glass to the brim. We started gently adding pennies, one at a time waiting for the water surface to be calm before we placed another one on there.

The set-up

Water careful filled to the brim

The adults showed J how to do it.

J was so excited to do it himself, but three year olds aren't super careful and like when things splash.

Gently add pennies 1 by 1

In general: as temperature increases, surface tension should decrease (making it harder to stick together). Our hypothesis (educated guess) was that the colder glasses of water would hold more pennies.

Our results:

Cold glass: 26 pennies (J)
Room temp glass: 25 pennies (Big J)
Hot glass: 18 pennies (Mommy)

Things to discuss:
*What shape does the water make after adding pennies?
*How many pennies did you get into the cup before it spilled?
*How did the temperature affect the surface tension in your experiment?
*Which cup of water held the most pennies?
*Repeat the experiment with different liquids (ex. juice or isopropyl (rubbing) alcohol -under extremely close adult supervision). How do the different liquids act when the pennies are placed in them? Why do you think that is?

Things that could have affected our results:
*Cups might not have been filled to the same levels.
*We didn't place the pennies in gently enough, causing a splash.
*Pennies weren't fully dry between cups (adding more water).
*Someone could have accidentally bumped the table (it happens with a three year old).
*Pennies had various amounts of dirt on them. A piece of grass ended up in the cold cup, which could have affected the experiment.
*Each person did pennies in a different glass. Ideally, we would repeat the experiment with the same user for all three glasses, but we're not that patient around here and everyone wanted a turn.

Happy experimenting!