Naked Science Forum
General Science => General Science => Topic started by: paul.fr on 16/02/2009 09:33:54
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you will need:
- a drinking glass
- 3-4 wooden toothpicks or match sticks
- a candle
- knife
- water
Fill the glass about half-full of water. Break each toothpick into 4 pieces. Carefully cut several chunks of wax from the candle. Each piece should be about 1/2 inch across.
Drop the pieces of wood into the water, one by one. Watch what happens. The pieces will float around until they get close to the side or another piece of wood. Then they will stick there. Soon, all the pieces of wood will be along the sides of the glass. Now drop in some chunks of wax, one by one. They will also clump together, but they will gather in the center of the glass!
Why?
Every molecule of water is made up of two atoms of hydrogen and one atom of oxygen. Both hydrogens are on the same side of the oxygen, and this gives the water molecule an electric charge. The end with the two hydrogens has a positive charge, and the opposite end has a negative charge.
These charges act much like magnetic charges. Opposites attract, which causes water molecules to stick to each other. It also causes them to stick to other polar substances. The wood is mostly made up of cellulose, which is also polar, so water sticks to it.
That stickiness causes the water to climb up the surface of the wood. When two pieces get close, the water clinging to the sides causes the water level between them to be higher. The pieces of wood float towards the higher level and move together. Since wood is "wet-able", it will float towards the highest water level.
Water clings to the side of the glass in the same way, so the wood floats towards the sides.
The wax is non-polar. Instead of sticking to the wax, the water bows downwards under it. This causes the chunks of wax to clump with other chunks of wax, and causes them all to drift towards the lowest point on the surface of the water. Since the center is the lowest point, the wax tends to wind up there.
Now for the challenge. The wood is along the edges of the glass, and the wax is in the center. Your task is to get them to switch places. You want the wax along the edges of the glass and the wood in the center. You may add things to the water, but you may not push on the pieces. Can you do it? Of course you can. You have a week to play with it and figure out the solution.
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Is it as simple as adding some liquid soap to break the surface tension?