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Name: Pearl
Status: student
Grade: 4-5
Location: PA
Country: N/A
Date: September 2006

I would like to know why pepper on water goes to the side of a bowl when you add soap to it?

The detailed answer is more complicated than you need. The "simple" answer is that the surface tension of the pepper particles is less than "pure" water (about 72 ergs/cm^2), so the pepper particles tend to float on the water's surface. When you add a drop of soap, you greatly reduce the surface tension of the water near the point at which you add the soap. Initially, this causes a repulsion of the pepper particles and the particles tend to follow the high surface tension areas. If you shake the solution, you will find that the pepper particles get "wetted" by the soap solution and the pepper will disperse in the soap solution. Similarly, if you add the drop of soap down one side of the floating pepper particles, you will observe them moving away from the point of addition, and collecting on the side opposite where you added the drop of soap.

Vince Calder


Adding soap to water changes its surface tension properties. Surface tension results from the fact that the water molecules prefers to stick to itself (the other water molecules) than to the air above it. So a kind of "skin" forms on the water surface where the water molecules are holding onto each other tightly and not interacting with the air above it very much. Soap molecules generally are long molecules with one end capable of attaching themselves to molecules like water, and the other end attaching themselves to different molecules like those found in air. When you add soap to water, the water molecules now attach themselves to one end of the soap molecules and the air molecules can attach themselves to the other end of those soap molecules. The result is that because the water molecules are now bridged to the air molecules by the soap molecules, the surface tension decreases in intensity, the "skin" effect is weakened.

But, the pepper is still sitting on parts of the water surface that had surface tension (the soap molecules has not gotten to those parts yet). So this parts of the water are pulled by the still strong surface tension away from the area where the soap was added. You can imagine this as a sheet of rubber that is under tension (stretched) and the tension is released (from where the soap was added) and as a result the rubber shrinks away from the loss of tension. The pepper which is riding on the parts that still have the surface tension therefore ride along as the skin (like the shrinking rubber) snaps away from the area where there is soap.

Greg (Roberto Gregorius)

Dear Pearl:

I will let the other scientists answer your question, but you can have some fun if you try this.

Fill a glass with water and let it sit for a minute or two. Take a small piece if toilet paper and drop it on the surface of the water so it floats. Carefully put a sewing needle on the toilet paper. You should now have a toilet paper "raft" with the needle "passenger" on top. You can wait until the paper sinks or you can carefully poke around the edges of the toilet paper until it sinks.

Either way, you will have a needle floating on water! Do you think this has anything to do with how some insects can "walk on water"?

Once the needle is floating, add a drop of detergent to the water. What happens to the needle? Why do you think it happens? Does this give you a clue as to why we use detergent to get clothes clean?

R. Avakian

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