Undeniable Friday: Un-dissolving Dye

Friday, September 01, 2006

It's Undeniable Friday! Every Friday, in addition to an odd little factoid, UndeniableFacts.com gives you a fun puzzle, illusion, or activity to enjoy and share.

Today: Un-dissolving Dye

The effect:
You stir a glass of colored water with a simple wooden stick and, contrary to expectations, the dye and water separate, leaving the water clear, and the dye at the bottom of the glass.



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Materials:
Wooden or bamboo stick (I used a plain chopstick)
Candle wax (not beeswax; too many electrolytes from the hornets' abdomens)
A pot or saucepan
3 tin cans
1 tbsp salt
1 tbsp vinegar powder (or other acidic powder)
1 tbsp baking soda (or other alkaline powder)

The setup:
This requires the use of a stove. Kids, get your parents to help!

Divide the wax into three equal portions. You will be melting three batches of wax, so put one portion into each can. Put enough water into the pot so that the water level comes to about half way up one of the cans. Turn on the stove and wait for the wax to begin to melt. Once it begins to melt, sprinkle the vinegar powder into the can, trying to distribute it evenly. Once the wax has completely melted, and you can no longer see bumps on top from the vinegar powder, dip the wooden stick in several times to get a good thick coat of wax on it. Set the stick aside, and carefully remove the can from the pot (use an oven mitt or tongs!) and throw it away.

Now repeat this process two more times, using the salt instead of vinegar powder for the second batch, and the baking soda for the third batch.

Once the stick cools, your work is done. Try a test run: mix some food coloring with some water and then stir it with your wax-coated stick. It's like magic!

The explanation:
Water is a polar molecule. This means that one side of the molecule has a positive charge, while the other side has a negative charge. Because of this, water can only dissolve other polar molecules. The dye in food coloring is a polar molecule, which is why it dissolves.

The key to this trick is to temporarily force the molecules in the dye to become non-polar. The wax-coated stick does this because the wax is "doped" with three different ionic compounds. The one in the center has a positive charge, while the one on the outside has a negative charge. The result is a material which acts like a polar solution, but whose "molecules" cannot easily turn around.

There's something else I didn't tell you about water molecules: their centers are also negatively charged. Remember, in the world of physics, opposites attract. This means that the water molecules are weakly repelled by the negatively charged exterior layer of the wax. More balanced polar molecules, like those in the dye, are not repelled, and so the net effect is that they move towards the wax. As they do so, their electrical imbalance is satisfied, which means they act like non-polar molecules. As a result, they precipitate from the water, allowing you to separate the solution.