HI- My 9 year old son would like to know why a certain science experiment works- when you put pepper in a bowl of water, the pepper floats. When you insert a bar of soap, the pepper is immediately repelled by the soap and moves to the edges of the bowl. Can you give us an explanation for this? Thank you! Janet and Jake-
What a great experiment! Before thinking up possible explanations, we had to try it for ourselves. We were surprised at how dramatic the effect was. We sprinkled pepper on the surface of water in a bowl. After a tiny drop of liquid soap was placed on the water in the center of the bowl, the pepper shot off toward the sides of the bowl, and we said “wow!”
I had my suspicion as to what was going on, but I thought I would let my fiance, Tim, who is not a chemist but an engineer, mentally work through it. “Is there something in pepper that is repelled by soap?” He asked me.
I could not imagine, and said I did not think so. I think pepper is the choice material for this experiment simply because it is black and shows up well. We repeated the experiment with flour and other floating materials on the surface of the water and it worked the same way. So there is nothing special about using pepper.
I told Tim my suspicion. The pepper is not repelled by the soap. That is an illusion. I know that water molecules are highly attracted to each other. Think of the water molecules as little magnets that stick to each other*.
Now imaging you have a bowl full of identical molecules that all stick pretty well to one another (that’s the water molecules, but this phenomenon also works for other molecules that attract one another readily.) When you have sticky molecules like this in a container, there is a difference between a molecule at the surface of the liquid, and one inside of it:
A molecule inside the liquid is surrounded by identical magnet-like molecules on all sides, so it is pulled in all directions at once, and it feels no net pull in any direction. However, a molecule at the surface has no identical molecules above it to attract it. It feels a net force downwards, because the only identical molecules that attract it are below it. This phenomenon at the surface of a liquid made of sticky molecules is called surface tension. Water has a high surface tension, and this creates something like a skin of water molecules on the surface of the liquid, which feel a net downward pull. Water bugs can skate on the surface tension of water.
Now, soap destroys the surface tension of water. Soap is any sort of molecule that has one end that attracts water (usually because it has a positive or negative charge and water loves charges), and another end that is attracted to grease or oil, so it causes oil and water to mix. Now, there is no oil in our experiment, but that is how soap works to clean oil off of things.
When you add the soap to our experiment you are adding all these great big molecules that are attracted to water, but are big and bulky and get in between water being attracted to water. It physically gets in the way of water’s ability to interact with other water molecules, which is what creates surface tension.
The surface tension remains strong wherever there is NO soap. The pepper can only stay afloat in those regions. This makes it appear as if the pepper were repelled by the soap, but really, the pepper just stays afloat on the skin of water, which “pops” like you might pop a balloon, and the skin of water scoots away from the soap, carrying the pepper with it.
Tim is usually pretty good at challenging my pronouncements, which, though annoying, helps keep me on my toes and keeps me from making outlandish statements when I am off track. He suggested that perhaps the soap floats on the water, and pushes the pepper away. But I told him that the soap should not float on water—it does have a water attracting end, and (as long as the water is not hard) should dissolve in the water. It will not float.
“But isn’t the soap lighter than water?” He persisted. It does not matter, I told him, what the densities of two liquids are if they mix. My students often encounter this misconception. They think that if two liquids have different densities, one will float on the other. But that is only true if they do not mix, like oil and water. Ethanol (alcohol) and water certainly have different densities, but I would alarmed if they separated and formed layers in an alcoholic beverage. They mix completely. I managed to convince Tim with this final argument.
If what you say is true, I suggested to Tim, then oil, which does float on water, would have the same effect as the soap. We tried the experiment again with oil instead of soap, but the pepper just stayed in place. So it was not a displacement pushing the pepper away from some sort of film on the top. Instead, pepper can only float where water has surface tension, and soap destroys the surface tension.
Finally, we amused ourselves by floating a pin on the surface of water. It was easier than I thought. Even though the pin is heavier than water, you can get it to float on the surface tension of water. We laid it on its side using a pair of tweezers. We added some liquid soap to the water and “Mr. Floatie” as Tim affectionately termed the pin, ultimately sank because the soap broke the surface tension.
*Polar and nonpolar molecules:
Actually, the reason why a water molecule sticks fairly well to another water molecule is because water molecules are “polar”.
Molecules that are polar have a permanent positive region and a permanent negative region on them. They reverse their orientations with respect to each other and stick to each other because opposite charges attract.
Water has a special type of polar attraction for itself which is especially strong and gets the term “hydrogen bond”; but that is a confusing term since it isn’t a real bond, it is nowhere near as strong as the real bonds that hold the water’s hydrogen atoms onto its oxygen. Real bonds are relatively tough and weld one atom to another to create a molecule. But you can break the much weaker hydrogen “bonds”-- these attractions of water molecules for other water molecules-- by just dragging your finger through a bowl of water, separating water molecules from each other temporarily, so you see that it isn’t that hard to break a so-called hydrogen bond.
However, the attraction of water for water is strong enough that a water droplet form a sphere, because the water molecules all attract themselves so well. Oil is not polar, and a drop of oil does not, on its own, round up into a sphere, it flattens out on a surface. So oil is not attracted to other molecules very well. Oil has no permanent negative or positive regions on it, it is called nonpolar. Oil (nonpolar molecules) and water (polar molecules) do not repel each other as is commonly thought. They are simply not very attracted to each other. Oil has no tempting charges on it for water to stick to. The water thus sticks to itself and excludes the oil. Water IS attracted to oil, just not as well as it is to itself. Oil going into water is like a person going to a party where no one talks to them. It isn’t that no one likes them. It’s that everyone there likes everyone else, better.