Education
Attraction
and Repulsion -
Picky toothpicks
Materials
Procedure
A
Arrange the
toothpicks in a circle in the bowl of water.
Step 2: Place the
sugar in the center of the circle. Watch as the toothpicks are
drawn toward the center of the bowl.
B
Step 1: Change
the water in the bowl and arrange the toothpicks in a circle
again.
Step 2: Place the
soap in the center of the toothpicks as you did with the sugar
cube. Watch as the toothpicks move away from the center of the
bowl.
C
Step 1: Change the
water in the bowl and arrange the toothpicks in a circle again.
Step 2: Add a drop
of vegetable oil and see what happens.
Step 3: Place the
soap in the center of the toothpicks and record your observation.
Surface tension is
the tension of the surface film of a liquid. It is caused by the
attraction of the particles in the surface layer by the bulk of
the liquid. Surface tension forms an "invisible skin" in water
that is enough to support an object that you would think would
sink.
In this case, the
sugar sucks up water, creating a current that carries the
toothpicks with it toward the center of the bowl. The soap,
however, gives off an oily film that spreads outwards, the oil
also makes the toothpicks move outwards. This film weakens the
surface tension of the water and carries the toothpicks away with
it. When you combine the oil and the soap, you will notice that
the toothpicks move outwards even faster because the soap repels
oil.
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