What Arises When Water Turns to Ice?
Why does water expand when it
cools?
Water is peculiar. When most
substances change from liquid to
solid form, they shrink and become
denser, their molecules packed most
closely together. But when water
changes from a sloshy liquid to solid
ice, it expands, becomes less dense.
Which is why ice floats to the top of
your Coke, rather than sinking like a
stone to the bottom.
At normal
atmospheric pressure, molecules
usually behave in predictable ways
as their temperature changes.
Molecules fly apart into a gas when
heated, condense into a flowing liquid
when cooled, and shrink into a frozen
solid when chilled still further. The
changes in state parallel changes in
energy: from high energy to medium
energy to barely jiggling. Boiling
water expands into a gas (steam) and
wafts off into the kitchen. But we
also see water expand when chilled
in the freezer. An ice cube tray filled
to the rim the night before overflows
with big cubes of ice in the morning.
Water starts out behaving normally.
As its temperature drops, water
obediently shrinks together - until it
reaches 4 degrees Celsius (39 degrees
Fahrenheit). Then, amazingly, water
reverses course, its volume slowly
increasing as it chills. When water
finally freezes, at 0 C (32 F), it
expands dramatically. Scientists say
water's quirky behavior is caused by
the shape of its molecule and by how
its molecules bond to one another.
Each water molecule is two hydrogen
atoms bonded to one oxygen atom
(H2O). Because of how the atoms
share electrons, a water molecule is
slightly positively charged at the
hydrogen
atoms,
and
slightly
negatively charged at the oxygen
atom. The molecule's charged ends
attract the oppositely charged ends of
other water molecules ("hydrogen
bonding").
In liquid water, as
molecules slip-slide past each other,
bonds form, break and re-form. But
by the time water has cooled to 4 C,
the molecules' energy has dropped
enough that they are very near one
another. So each H2O molecule forms
more stable hydrogen bonds, with up
to four fellow molecules. By 0 C (32
F), the H2O molecules are snappily
lined up in a frozen crystal lattice, an
open hexagonal (six-sided) shape.
Unlike in liquid water, the molecules
in ice are held rigidly apart. That
means more empty space between
molecules, so frozen water occupies
more room. Result: Put 10 cups of
water in the freezer, take out nearly
11 cups of ice! Other real-world
results: Water pipes freeze and burst
in unheated houses. Water collects
in roadway cracks in winter, turns to
ice, and enlarges crevices into gaping
potholes. If frozen water weren't less
dense than liquid, there would be no
floating icebergs to sight off the bow
of a ship. There would be no skating
on ice- covered ponds, while fish and
other forms of life shelter in
insulated water below. If water froze
from the bottom up, much of Earth's
water would solidify in winter, and
life might be impossible.
Kathy
Wollard
is
a
contributor to Newsday.
regular
What Arises When Water Turns to Ice?
1. What happens to most substances when they change from a liquid to a solid form? Why is water’s behavior considered
unusual?
2. What is the temperature at which liquid water starts to expand as it freezes and what mechanism causes this?
3. What are some real life consequences/outcomes due to water’s unusual behavior?
4. If you have a glass of water filled to the “tippy top” with water and ice, what will happen to the water level in the glass
when the ice melts?
5. Glaciers store ice on land and icebergs store ice in the ocean. Global watering is causing both glaciers and icebergs to melt.
How will the meting of these objects affect the sea levels around the world?