Pepper, Soap, and Surface Tension
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.
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)
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?
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Update: June 2012