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Water balancing trick

By David, 29 November 2018 Activity

A five cent coin with a convex drop of blue water covering the surface.Here’s a balancing game, but with a bit of practice you will always win! Why? Because you’ve set up a clever trick.

You will need

  • 2 five cent coins
  • 2 eye droppers
  • Cup of water
  • Dishwashing detergent
  • A friend to trick

What’s the game?

In this game, take turns to put drops of water on your coins. Whoever spills their water first, loses!

What to do

  1. Practice makes perfect! Test all the steps below a few times to try the trick before you invite someone to play with you.
  2. An index finger tip, shiny with detergent.Without anyone seeing, put a very small drop of detergent on your index finger.
  3. Two five cent coins in the palm of a hand.Keeping the detergent away from the coins, put both coins on the palm of your other hand.
  4. Find your friend, and explain that you’ll both be taking turns to put drops of water on your coins. The first to spill their water loses.
  5. Ask your friend to pick one of the coins.
  6. Touching a five cent coin with a fingertip.Put their coin on the table and secretly touch the top face of the coin with detergent from your index finger.
  7. Put your coin on the table, making sure you don’t get any detergent on it.
  8. An eye dropper dripping blue liquid onto a five cent coin.Choose an eye dropper each. Suck up some water with your eye droppers and start playing. Take it in turns to place a drop of water onto your coin. If you managed to get detergent on their coin, you should win every time!

 

What’s happening?

You might be surprised to find how many drops of water you can fit on a coin without it spilling. With care, you can pile the water millimetres high. This is because water ‘sticks’ to itself. Attraction between water molecules causes the water droplets to clump together in a larger blob.

Detergent contains surfactants, chemicals that battle against water clumping. A surfactant molecule usually has two parts. One end is attracted to water, and the other end is attracted to some other chemicals.

Surfactants tend to move to the water’s surface. This makes the water more attracted to other substances, including air. This new attraction balances against water’s natural clumping.

The result is that drops of water don’t hold together as tightly with a surfactant around. So the coin with detergent won’t balance as much water on top.

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