превращается в лед даже при минусовой температуре..
Nova: Mystery of the Megaflood.
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При этом развлекаться мы развлекаемся, а даже не все знают, при какой
температуре замерзает вода. Давайте проделаем эксперимент.
... Если изменить атмосферное давление,
то изменится и температура замерзания. .... Обычная питьевая вода
превращается в лед при температуре 0 градусов по Цельсию.
Не только ДАВЛЕНИE влияет на точку замерзания воды и
превращение ее в лед.
Стерильная(т.е лишенная добавок,соли и минералов) вода
замерзает при гораздо более
также ,как и очень соленая вода.
Supercool: Water doesn't have to freeze until -48 C (-55 F)
November 28, 2011
University of Utah
We drink it, bathe in it and are made mostly of it,
yet common water poses major mysteries. Now, chemists may
have solved one enigma by showing how cold water can get
before it absolutely must freeze: 48 degrees below zero
Celsius (minus 55 Fahrenheit).
Regular ice crystals (red) are mixed with
“intermediate ice” (green) near the end of the process
of crystallizing from supercooled water. University
of Utah chemists used computers to determine that
water, which doesn’t necessarily become solid at its
32 degrees Fahrenheit freezing point, actually can get
as cold as minus 55 F before it must freeze.
Credit: University of Utah
We drink water, bathe in it and we are made mostly of
water, yet the common substance poses major mysteries.
Now, University of Utah chemists may have solved one
enigma by showing how cold water can get before it
absolutely must freeze: 48 degrees below zero Celsius
(minus 55 Fahrenheit).
That's 48 degrees Celsius (87 degrees Fahrenheit)
colder than what most people consider the freezing
point of water, namely, 0 C (32 F).
Supercooled liquid water must become ice at minus 48 C
(minus 55 F) not just because of the extreme cold,
but because the molecular structure of water changes
physically to form tetrahedron shapes, with each water
molecule loosely bonded to four others, according to
the new study by chemists Valeria Molinero and Emily Moore.
The findings suggest this structural change from liquid
to "intermediate ice" explains the mystery of "what
determines the temperature at which water is going
to freeze," says Molinero, an assistant professor at
the University of Utah and senior author of the study,
published in the Nov. 24 issue of the journal Nature.
"This intermediate ice has a structure between the full
structure of ice and the structure of the liquid," she
adds. "We're solving a very old puzzle of what is
going on in deeply supercooled water."
However, in the strange and wacky world of water, tiny
amounts of liquid water theoretically still might be
present even as temperatures plunge below minus 48 C
(minus 55 F) and almost all the water has turned
solid -- either into crystalline ice or amorphous
water "glass," Molinero says. But any remaining
liquid water can survive only an incredibly
short time -- too short for the liquid's properties
to be detected or measured.
How and at what temperature water must freeze has more
than just "gee-whiz" appeal. Atmospheric scientists
studying global warming want to know at what temperatures
and rates water freezes and crystallizes into ice.
"You need that to predict how much water in the atmosphere
is in the liquid state or crystal state," which
relates to how much solar radiation is absorbed by
atmospheric water and ice, Molinero says. "This is
important for predictions of global climate."
A Strange Substance
Liquid water is a network of water molecules
(each with two hydrogen atoms and one oxygen atom)
held loosely together by what is called hydrogen
bonding, which is somewhat like static cling. Molinero
says that depending on its temperature and pressure,
water ice has 16 different crystalline forms in which
water molecules cling to each other with hydrogen bonds.
Molinero says that "what makes water so strange is
that the way liquid water behaves is completely
different from other liquids. For example, ice floats
on water while most solids sink into their liquid
forms because they are denser than the liquids."
Water's density changes with temperature, and it is
most dense at 4 C (39 F). That's why fish survive under
ice covering a pond by swimming in the warmer, denser
water at the bottom of the pond.
But the property of water that "is most fascinating is
that you can cool it down well below 32 degrees Fahrenheit
[zero Celsius] and it still remains a liquid," says Molinero.
Liquid water as cold as minus 40 C (minus 40 F) has
been found in clouds. Scientists have done experiments
showing liquid water can exist at least down to minus
41 C (minus 42 F).
Why doesn't water necessarily freeze at 0 C (32 F) like
we were taught in school?
"If you have liquid water and you want to form ice, then
you have to first form a small nucleus or seed of ice
from the liquid. The liquid has to give birth to ice,"
says Molinero. "For rain, you have to make liquid from
vapor. Here, you have to make crystal [ice] from liquid."
Yet in very pure water, "the only way you can form a
nucleus is by spontaneously changing the structure of
the liquid," she adds.
Molinero says key questions include: "Under which
conditions do the nuclei form and are large enough
to grow?" and "What is the size of this critical nucleus?"
Computing What Cannot Be Measured
Molinero says that "when you cool down water, its
structure becomes closer to the structure of ice,
which is why the density goes down, and this should be
reflected in an increased crystallization rate."
Supercooled water has been measured down to about minus
41 C (minus 42 F), which is its "homogenous nucleation
temperature" -- the lowest temperature at which the ice
crystallization rate can be measured as water is freezing.
Below this temperature, ice is crystallizing too fast
for any property of the remaining liquid to be measured.
To get around the problem, Molinero and chemistry doctoral
student Moore used computers at the University of
Utah's Center for High Performance Computing. The
behavior of supercooled water was simulated and
also modeled using real data.
Computers provide "a microscopic view through simulation
that experiments cannot yet provide," Molinero says.
Previous computer simulations and modeling were too
slow and had to last long enough for the freezing process
to occur. And it was necessary to simulate thousands of
nucleation events to make valid conclusions.
Molinero and Moore devised a new computer model that is
200 times faster than its predecessors. The model
simplified the number crunching by considering each
three-atom water molecule to be a single particle
similar to a silicon atom and capable of sticking
together with hydrogen bonding.
Even so, it took thousands of hours of computer time
to simulate the behavior of 32,768 water molecules
(much smaller than a tiny drop of water) to determine
how the heat capacity, density and compressibility
of water changes as it is supercooled, and to simulate
how fast ice crystallized within a batch of 4,000
The Birth of Ice
The computers helped Molinero and Moore determine how
cold water can get before it reaches its theoretical
maximum crystallization rate and must freeze. The
answer: minus 48 C (minus 55 F).
The computers also showed that as water approaches
minus 48 C (minus 55 F), there is a sharp increase in
the proportion of water molecules attached to four
others to form tetrahedrons.
"The water is transforming to something else, and this
something else is very close to ice," says Molinero.
She calls it intermediate ice.
If a microscopic droplet of water is cooled very fast,
it forms what is called a glass -- low-density
amorphous ice -- in which all the tetrahedrons of
water molecules are not lined up to form perfect
crystals. Instead, low-density ice is amorphous
like window glass. The study found that as many
as one-quarter of the molecules in the amorphous "water
glass" are organized either as intermediate ice or
as tiny ice crystals.
When water approaches minus 48 C (minus 55 F), there
is an unusual decrease in density and unusual increases
in heat capacity (which goes up instead of down) and
compressibility (water gets easier to compress as it
gets colder, unlike most liquids). These unusual
thermodynamics coincide with liquid water changing to
the tetrahedral structure.
"The change in structure of water controls the rate
at which ice forms," Molinero says. "We show both the
thermodynamics of water and the crystallization
rate are controlled by the change in structure of
liquid water that approaches the structure of ice."