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Some choose
not to believe it, but this phenomenon can be
demonstrated in your own freezer under the proper conditions.
If you have
two pails filled with equal amounts of water, one hot and one cold,
and you set them
out in the snow on a cold winter's morning, what happens?
What
Should Happen
Any reasonable
person would think that the hot pail would take longer than the
cold pail to freeze. After all, the hot water needs extra time to
reach the same temperature as the cold pail. When the hot pail finally
does reach the same temperature as the cold pail initially was,
the cold pail should already be frozen.
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What
Actually Happens
There are
several things that help the hot pail freeze faster than the cold
pail. Here are what is thought to be the most significant factors:
Layer
of ice forms on the top of the cold water.
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The hot
water is more likely to be supercooled. This means that the
hot water's temperature is more likely to cool to temperatures below
zero degrees Celsius. In the cold non-supercooled water, ice crystals
form and float to the top, forming a sheet of ice
over the top of the water, creating an insulating layer between
the cooler air and the water. This ice sheet also stops evaporation.
In the hot water that has become supercooled (thus, no longer hot)
the water, when it does freeze, freezes throughout, creating more
or less of a slush before freezing solid.
Why is hot water more
likely to be supercooled? Because hot water is less likely to contain
tiny gas bubbles. Gas bubbles form from dissolved gasses as the
water cools. When the hot water was heated, these dissolved gasses
may have been driven out. In cold water, ice crystals use the tiny
bubbles as starting points for formation (in physics, we call them
nucleation points). But in the hot water, there are no bubbles,
so there aren't as many starting points for the ice crystals.
Dissolved
gasses also lower the freezing point. Since heated gas is less likely
to contain dissolved gasses, it's more likely to freeze first.
Water
in the hot water pail evaporates at a much faster rate than the
cold water. This does two things.
First, the
process of evaporation is endothermic, which means it takes energy
for something to evaporate. As a molecule of water evaporates, it
leaves the surface of the water and flies into the atmosphere. Thus,
in simplified terms, the molecule converted heat energy into kinetic
energy (energy of motion). Since the hot water evaporates quicker
than the cold water, it loses heat energy quicker than the cold
energy.
Second,
since some of the hot water evaporates away, there is less water
left to have to freeze.
The hot
water pail will melt the surrounding snow. Later, as it begins
to freeze, the snow around the pail will freeze back so that it
more closely "touches" the pail. The cold water pail is
then only sitting in fluffy airy snow, while the hot water pail
is in a form fitting ice-crust. The ice-crust will obviously conduct
the cold better that the airy snow.
Other factors,
such as convection currents (the movement made as hot water rises
while cool water sinks) may or may not play a role in this odd phenomenon.
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