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This Week's Experiment - #259 Slippery and Sticky Ice
We have been watching the Winter Olympics, so I decided to do an experiment
with ice. It is a simple one, but it ties in well with the Olympic sports.
You will need:
two ice cubes
a wash cloth or paper towel
Pick up the two ice cubes. Look for the smoothest side on each cube. This
will probably be the top if the cube was made in an ice cube tray, or the
side if it came from an ice maker. If you don't like getting your fingers
cold, you can use the cloth to hold the ice. Rub the two smooth sides
together and you should notice that they slide across each other very easily.
It is easy to see why it is so easy to slip on an icy sidewalk.
Next, hold your hand out flat, with your palm upwards. Cover your hand with
the cloth and then place the two ice cubes in the center of your palm, with
the two flat surfaces touching. Now, squeeze the two cubes together firmly
for a few seconds. Release the pressure and examine the ice. They are now
stuck together!
How can the ice be slippery one minute and sticky the next? This is
something that has interested scientists since the time of my hero, Michael
Faraday. In fact, Faraday did quite a bit of experimentation on this very
subject. Many scientists claimed that pressure on the ice cubes caused the
ice to melt and then when the pressure was released, it would refreeze.
Faraday thought that ice always had a thin layer of water on its surface and
that when you pressed the ice pieces together, the parts that were firmly in
contact were no longer part of the surface. This allowed the layer of water
to freeze, connecting the two pieces.
Since then, it has been shown that Faraday was correct. The layer of water
gives ice its slippery surface. It can also refreeze if it is between two
pieces of ice that are firmly in contact. The thickness of this layer of
water depends on the temperature. The colder it is, the thinner the layer.
A thinner layer means less "cement," which is why wet snow makes better snow
balls than very cold, dry snow. It is one of the reasons the sports
announcers talk about how fast the ice is and the quality of the snow.
Don't waste the ice cubes from your experiment. Put them in a glass and
cover them with something good to drink. Pop some popcorn and settle back to
watch all the science in these sports.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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