Why Ice Floats

I can see them now, thousands of square dancers, each one pairing up with
a partner for a few seconds and then moving on to someone else for an equally
brief time. Only when the music slows down do the dancers spend more time
together with an individual partner.
Strangely enough, that’s how I picture the bonding between water
molecules in a glass of water. (Stick with me here, okay? By the time we’re done,
you’ll hopefully understand why ice floats and also, why ice is slippery.)
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A water atom has two hydrogen atoms and one oxygen atom. That’s why it’s
called H 2 O. The oxygen atom attracts the electron cloud from around the
hydrogen, leaving the hydrogen atom with a slight positive charge. It can use this
to attract the oxygen atom from another molecule of water. This is called hydrogen
bonding. We can see how this works from the figure below, provided by The
Interactive Library web site.
In the figure, each white ball represents a hydrogen atom and each red ball
represents an oxygen atom. The green dotted lines represent hydrogen bonding
between water molecules.
In square dancing a man and a woman become a new “couple” every few
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seconds. Water molecules do the same thing, except each water molecule forms a
new bond with another water molecule a million-million times every second!
When water approaches its freezing temperature, the dance slows down
and water molecules begin to form more lasting partnerships. When that happens,
the chaotic pattern becomes more ordered, until the freezing point is reached.
Shown below are two structures from The Interactive Library web site.
(Note: Ice III is one of several forms of ice that form under various
conditions of temperature and pressure.)
Do you notice the empty spaces in the ice structure as compared to the
liquid water structure? This shows that water molecules in ice are not as tightly
packed as they are in liquid water. In other words, a cubic foot of ice will have
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fewer water molecules that a cubic foot of water. We usually state this by saying
that the density of ice is less than the density of water.
Water has a density of 1.0 grams per cubic centimeter. Ice has a density of
.92 grams per cubic centimeter; oak wood has a density of .71 grams per cubic
centimeter.
Materials that have a lower density than water float in water. Therefore ice
floats. About 10 percent of ice is above water.
So why should you even care that ice floats?
Consider a pond about to freeze. Because the outside air is below freezing,
the coldest water will be near the top where all the cold air is.
If cold water about to freeze was more dense than the warmer water below
it, then it would sink to the bottom of the pond. Ice would then form on the bottom
of the pond. Because that ice would not feel the full warmth of the sun during the
summer, some ice on the bottom might remain all through the summer.
In time the entire pond might become frozen each winter. In the summer
perhaps only a few inches of ice near the top of the pond would melt.
Because so much water would be locked up in the form of ice for years, few
clouds would carry water to the rest of the planet. In time this would mean the
death of most all life on the Earth. But that doesn’t happen because ice floats!
What is even more remarkable is that water is one of only a few substances
that expand when going from a liquid to a solid state. Boy, did we luck out! !
Second question: Why is ice so slippery?
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Even though ice is a solid, it’s not the same as other solids. For instance,
concrete, wood and glass are solids and yet we don’t skate on them.
According to The New York Times web site, the current explanation of why
ice is slippery can be introduced by the figure below from the 2006 New York
Times:
According to the New York Times, “…water molecules at the ice surface
vibrate more, because there are no water molecules above them to help hold them
in place, and thus they remain an unfrozen liquid even at temperatures far below
freezing.”
In other words, all ice at any temperature contains a thin liquid-like layer
that will never freeze because the ice crystals below them are not able to force
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them into the ice structure.
Back to our square dance analogy. It’s like all the square dancers are sitting
around hexagon shaped tables having refreshments except for a small group that
are doing line dancing near the edge of the dance floor. While everyone else is
sitting around, they’re still bonding with new partners just like always.
According to the New York Times, “Michael Faraday in 1850 proposed this
idea after performing this simple experiment. He pressed two cubes of ice against
each other and they fused. Faraday argued that the liquid layers froze solid once
they were not on the surface. But because the layer was so thin, it was hard for
scientists to see.”
Based on the results of an experiment performed in 2000 by the Institute of
Physical Chemistry, I estimate this liquid-like layer to be about 100 layers of water
molecules thick. If any of you have a better number though, I’d be pleased to see
it.
So what we’re saying is that the thin liquid-like layer which exists on the
surface of ice even when it’s very cold explains why ice is slippery. That probably
won’t give you much comfort the next time you slip and fall on the ice. My advice
for that is when the sidewalks are icy, wear boots and walk through the snow
because it’s not as slippery.
“And dosido, around you go!” See how much you can learn from square
dancing?
Weyland welcomes your comments and can be reached at
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jack.weyland@gmail.com