January 2018:

Materials:

Glass, Water, Pepper, Dish Soap

Experiment:

In a glass of water, lightly sprinkle a little pepper onto it. What do you see happen? Maybe some of the flakes sank and some of them floated? Now, touch your fingertip to the dishwashing soap and then using that finger, touch it to the surface of the water. What do you see happening now?

What’s happening?

In the November issue we talked about how polar water was and what effect soap has on water. This is another application of the same principle. First of all, most likely some of the pepper sank when you sprinkled it on the water. If something sinks in water we say it is more dense than water. It turns out that the density of pepper is about 1.5 g/cm³ while that of water is 1.0 g/cm³ (or g/mL). So if pepper is more dense than water why doesn’t it all sink? This is where surface tension comes into play. In the past year we’ve talked a lot about surface tension and that with water being as polar as it is, those water molecules like to hold onto each other and not let things go through them easily (that’s why water bugs are able to walk on water). I like to think of water as a mini magnet and those magnets hold onto each other. Because the water holds so tightly to itself, most of the pepper is able to float on water. Probably the first thing that you noticed though was that the pepper went scurrying to the edge of the glass when you touched the soap to the water. The soap is breaking the surface tension and so the pepper stays with the water that still has surface tension. When I say it lost its surface tension, what does that really mean? Again, the surface tension is caused by water molecules holding very strongly to each other and so the soap is disrupting those interactions by creating new interactions of the water with the soap which are not nearly as strong as the water-water interactions. We saw this in last month’s ChemShorts when we touched a toothpick with soap on it to a water droplet and the droplet lost its sphere-like shape. It’s possible that some of the pepper ended up sinking as the soap formed micelles around the pepper – although those pepper particles are really, really, really BIG! These micelles would allow the pepper to sink but most likely the water simply lost its surface tension allowing the more dense pepper to sink.

One last note, did you happen to notice how fast that pepper moved? I am amazed at how fast molecules in water move. It turns out that they move at approximately 1300 miles per hour or 590 meters per second! That’s fast.

To see another take on this experiment, check out the Marangoni effect at Khan Academy. https://www.khanacademy.org/partner-content/mit-k12/mit-k12-physics/v/the-marangoni-effect-how-to-make-a-soap-propelled-boat

References:

http://www.abc.net.au/science/surfingscientist/pepperscatter.htm

http://www.verticallearning.org/curriculum/science/gr7/student/unit01/page05.html

To view all past “ChemShorts for Kids”, go to:
http://chicagoacs.org/articles/article_category/1

- Paul Brandt