Water Can Remain Liquid Until 48°C/54°F Below Zero
Water Can Remain Liquid Until 48°C/54°F Below Zero
A study analyzes why H2O, in specific circumstances, does not freeze at negative temperatures
El País: El agua se puede mantener líquida hasta los 48 grados bajo cero
Alicia Rivera reporting from Madrid November 30, 2011
Water, so abundant in the universe, essential for life on the planet Earth, the principal component of the human body, omnipresent in oceans, glaciers, rivers, and the atmosphere, be it solid, liquid, or gaseous, continues to keep many of its secrets from scientists. For example, what determines the minimum temperature water can withstand before it becomes ice? Two scientists in the United States affirm they have found the answer and explain that supercold water can stay liquid until -48°C (-54.4°F), far removed from 0°C/32°F, normally considered freezing point.
In addition, they have found that the formation of water is not exclusively controlled by temperature; physical changes in the molecular structure of water play an essential role, as well. Emily B. Moore and Valeria Molinero, researchers in the Chemistry Department for the University of Utah at Salt Lake (USA), explain their research, based on modeling the processes of water on a computer, in the latest edition of Nature. “We’re solving a very old puzzle of what is going on in deeply supercooled water,” says Molinero.
Their research not only stands out in the field of basic science; it also has important practical implications. For example, knowing how and why water freezes is crucial for atmospheric experts studying the heating of the planet, because they need to determine how much of the water in the air is liquid and how much is crystallized, which has a notable influence on the amount of solar radiation that the terrestrial atmosphere absorbs. It is also key information for climate change models.
Liquid water is a network of molecules, each one formed by two atoms of hydrogen and one of oxygen (the classic H2O), united by hydrogen bonds. The Utah researchers explain that, depending on its temperature and pressure, ice has 16 crystalline forms that unite some molecules with others.
Water has strange qualities that make it act very differently from other liquids. For example, ice floats on water, while other substances become more dense when freezing and sink instead. For this reason, polar ice caps float on the ocean while fish swim in the more temperate water underneath them.
“One of the unsolved mysteries of water is what determines the minimum temperature to which water can cool before solidifying into ice,” write the researches in their article published in Nature.
Minimum freezing temperature is much lower than zero degrees: liquid water has been observed in the clouds at -40°C, and experiments have demonstrated that water can remain liquid at -41°C.
Scientists know that if water is in contact with another material or has impurities, these act as nuclei and induce the crystallization of ice; freezing normally occurs at zero. But pure water, without nucleating agents, can maintain a liquid state at very low temperatures before undergoing a change of state. “To create rain, you have to have liquid water apart from vapor; if you have liquid water and want to make ice, first you have to form a small nucleus or seed inside this liquid,” Molinero signals. “When the water is very pure, the only way to form a seed is a spontaneous change in the structure of the liquid.”
The problem, Nature distinguishes, is that it’s very difficult to study the so-called homogeneous nucleization of ice, as crystallization is very rapid at the moment of freezing. The crystallization of ice has been evaluated at 41 below zero, but below this temperature, the process is too fast.
What Moore and Molinero have done is investigate this process in extreme conditions through advanced computer modelization made off experimental data.
Although they have utilized a new modelization system for the freezing of water that is 200 times more rapid than those employed in previous studies, they needed thousands of computing hours to simulate the behavior of exactly 32,768 water molecules (much less than what forms a single droplet) to determine the changes (thermic capacity, density, and compression) of water upon supercooling and to simulate the velocity of ice crystallization.
This result shows that, at approximately -48°C, there is a notable increase in the proportion of water molecules bonded to four other molecules to form tetrahedrons. “The water is transforming into something else, and this something is very similar to ice, a kind of intermediate ice,” Molinero explains.
“An unusual drop in density is produced,” the researcher adds, “and an equally unusual increase in thermic capacity and compression capacity, which explains why water is easier to compromise as it freezes, unlike other liquids. This small thermodynamic current coincides with the changes of liquid water inside the structure of the tetrahedrons.”
In summary, the changes in the physical structure are what control the rate of the formation of ice from water, and forty-eight below zero is the lowest temperature water can be before it has to freeze.
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