Last week we started experimenting with food coloring, and I promised we
would continue this week. This one was an accident that caught my attention.
As I was putting a drop of food coloring into the bowl of water for last week's
experiment, something strange happened. There was some dried coloring on
the top of the bottle, and as I removed the top, some dried bits fell into the
water. What happened next was quite a surprise. The bits began to move
across the surface of the water, zipping along like little motor boats. As
they moved, they left a trail of color.
To try this, you will need:
some food coloring
a plate or saucer
a knife or fork
a bowl of water
Place several drops of food coloring onto the plate, and place it in the sunlight or a warm, dry place. Let it stay there until it is very dry. If
you scratch the surface with the knife and it smears, then it is not dry enough. You want to be able to scrape up small, solid chunks of coloring.
Place the bowl of water on a table or flat surface. Scrape the plate to
produce small, solid pieces of coloring. Carefully drop one or two of these into the bowl. Watch carefully. Some of the pieces will sink, or just sit there, but a few of them should move around the surface, like tiny motor boats. Why are they doing that?
Think back to experiment # 431 Scaring Pepper.
In that experiment, we saw that a tiny bit of soap could weaken the surface
tension of the water, letting the stronger surface tension of the water without the soap to pull the pepper grains to the side of the bowl. That is the same thing that is happening here. The coloring agent is weakening the surface tension of the water. If the chunk of dried color was round and smooth, it would weaken the surface tension in all directions, but with an irregular
chunk, more of the color will dissolve in one direction than in the others. The direction with the most dissolving color will have the weakest surface tension, so the chunk will be pulled in the opposite direction, just as the pepper was pulled in experiment # 431.
From Robert Krampf's Science Education Company
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