Surface tension is the attraction that molecules in a liquid have for each other. The water molecules at the surface of a puddle of water have more attraction to each other than they do to the air molecules above them, so they create a dome shape as they try to stick together. We have already done the science demonstration of counting how many drops of water will fit on a penny, so I thought we would take it a step further and make it an experiment!
The purpose of our experiment was to see which liquid has the most surface tension: water, soapy water, or vegetable oil.
The process was simple – we used a plastic pipette to drop our liquids onto a penny, and we counted how many drops of each the penny held before the liquid spilled off the side of the penny. To help ensure the best results, we used a clean penny each time.
Our results were interesting. I knew that the oil should have less surface tension than the water, but we had equal results – 23 drops for each. Soapy water had the least surface tension. We were only able to get 11 drops of soapy water on the penny.
While the boys were taking a break, I did a little googling to help explain our results! I found a great article on the penny experiment at Teach Engineering. The article explained that even though we were using the same type of pipette for each liquid (we used a clean one each time, but they were identical), the drops coming out of the pipette might be a different size. That totally made sense. If the drops of water were larger than the drops of oil, then we actually had more water on our penny than we had oil.
So I called the boys back over, and we tried putting one drop of each liquid on a plate side-by-side so that we could examine the drops.
The drops are (left to right) oil, water, and soapy water.
Using this method, it was obvious that water was the winner on surface tension! Soap molecules form chains which have one end that is attracted to water and one end that repels water, and so soap decreases the surface tension of water to about a third.
We actually tried again with just water on another day, and got totally different results, so now I wonder if the weather affects this experiment as well. The picture at the top was on a humid day, and then the photo below was a penny on a dry day. We fit 27 drops of water on this penny!
I had the boys write out all of the steps of the scientific method for this experiment. It was a really good experiment to write about because we had to re-work our results, and because we discovered some interesting things along the way.
For younger kids: Dropping the various liquids makes a great demonstration for preschoolers! They won’t be coordinated enough to drop the water accurately themselves, but it’s fun to watch how many drops fit on a penny. I know I was amazed! It’s also fun to drop water on a bird’s feather. Their feathers are coated in oil, and the drops of water roll right off!
For more fun with weather:
Demonstrate that air takes up space with a bottle, funnel, and clay – this is fun to watch!