SCARED PEPPER
PURPOSE
The purpose of this activity is to examine the behavior of pepper on the surface of water
when liquid detergent is introduced.
MATERIALS
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light-colored plastic bowl (unbreakable plastic, the size of a cereal bowl or larger,
and light-colored so the black pepper can be seen easily)
Water (a plastic soda bottle works well as a container)
Black pepper
Liquid detergent (a plastic film canister works well as a container)
Toothpick or cocktail pick
PROCEDURE
1. Fill the bowl half-full with water.
2. Sprinkle just enough black pepper over the water in the bowl to lightly cover the
surface of the water.
3. Dip the toothpick into the liquid detergent, and insert the detergent end of the
toothpick vertically into the center of the bowl of water.
4. Observe carefully what happens to the pepper.
EXPLANATION
Why does the pepper "run" quickly to the sides of the bowl, leaving the water in the center
clean? Water has a strong surface tension due to the unique attraction of water molecules for
each other. Water is a polar substance (one that has a positively charged end and a negatively
charged end) inasmuch as there is a partial positive charge on the hydrogen atoms and a partial
negative charge on the oxygen atom. Since unlike charges attract each other, water molecules
are attracted to each other, negative charge to positive charge. More complex attractive forces
called hydrogen bonds also bind the water molecules to each other.
Unlike water molecules in the interior of the liquid, which are bonded equally in all directions,
water molecules at the surface are drawn to each other in fewer directions (horizontally and
various angles downward) because air molecules lie above the water surface. With fewer total
bonds, each surface water molecule bonds more strongly with those water molecules that
surround it to the sides and below, almost forming a "skin" on the water surface. This "skin" or
surface tension of water is sufficient to hold the black pepper particles on top of it. With very
careful placement, one can even float a needle on the surface of water!
A characteristic of detergent molecules is that they have a partially charged polar end and an
uncharged nonpolar end. When detergent is introduced into the center of the bowl, the nonpolar
pepper particles are attracted more strongly to the nonpolar end of the detergent molecules than
to polar water molecules, and they "ride" the drop of detergent as it spreads rapidly over the
surface of the water. The pepper particles remain at the sides of the bowl until the detergent
forms a thin, even film over the whole water surface, allowing the pepper to spread back over the
surface later.
ADDITIONAL SUGGESTIONS
To determine the validity of the explanation (theory or hypothesis) above or to devise
their own explanation, encourage students to suggest additional experiments to test the
theory and explore why the pepper went to the sides of the bowl rather than to the
bottom. Don't spoon-feed the students with ideas. Some of the following experiments
and many more could be tried either in school or at home with parents participating.
Some of the following additional supplies may be needed as well as others.
Red food coloring
Thyme
Raspberry or strawberry gelatin
Cooking oil
1. Add more than 1 drop of detergent to see if the surface tension of the water is
decreased sufficiently to allow the pepper to sink through the surface. (The pepper
doesn't sink.)
2. Add a second drop of detergent to one side of the bowl where the pepper
accumulated to see if the pepper is riding the top of "clean water" which had not
sufficiently had its surface tension decreased by the detergent. (The pepper does
not sink; it was not riding clean water.)
3. Add detergent originally at one side of the bowl instead of in the middle. (The
pepper "runs" to the other side of the bowl.)
4. Add initially 2 drops of detergent simultaneously on opposite sides of the bowl.
(Two rings of pepper particles form, with a common side in the middle.)
5. Add red food coloring to the detergent to see if the detergent forms a film across
the entire surface of the water. (A film cannot be seen, but some of the red food coloring
sinks to the bottom of the bowl.)
6. Sprinkle thyme over the surface of the water to see if pepper was unique in its
behavior. (Thyme behaves in the same manner as pepper.)
7. Sprinkle raspberry or strawberry gelatin over the surface of the water to see if a
substance known to be attracted to water behaves in same as pepper. (Some
gelatin sinks; some floats and behaves in the same manner as pepper.)
8. Use several drops of cooking oil in place of the pepper in the activity. (The drops
coalesce or come together in the water initially, and then rapidly disperse when detergent is
added. Some of the detergent seems to disappear.)
9. After adding several drops of cooking oil to the water, sprinkle pepper onto the
surface of the oil "glob" which forms on the surface to see if the pepper is
attracted more strongly to the oil or the water. (The pepper stays with the oil glob when
the detergent is added.) Pour a small amount of detergent into a small bowl.
Sprinkle pepper over the surface of the detergent. Add a medicine dropper or
two of water to the bowl to see if pepper is attracted more strongly to water or to
detergent. (The pepper spreads out and stays with the detergent as the water forms a "pool" in
the center of the bowl.)
Have students bring their ideas together and decide what their results have shown.
REFERENCE
1. WonderScience, Fun Physical Science Activities for Children and Adults to
Do Together, James H. Kessler, editor, American Chemical Society,
Washington, DC, October, 1989, volume 3, number 3, p. 5; James H. Kessler
and Andrea Bennett, The Best of WonderScience, Over 400 Hands-on
Elementary Science Activities, Delmar Publishers, Albany, NY, 1997, p. 125.