Supercooling water
6 Jan 2012 by qwwqdccwwm.d qwwqdccwwm.d
Water can reach temperatures as low as -48°C before it absolutely must freeze say researchers in America, who believe their work may have important implications for the global climate.
Chemists from the University of Utah have been studying supercooled water to see what temperature it can reach before it freezes. At -48°C, water must become ice not only because of the extreme cold, but because its structure changes physically to form tetrahedron shapes, with each water molecule loosely bonded to four others.
Using a new computer model, Valeria Molinero and Emily Moore simulated the behaviour of 32,768 water molecules to determine how heat capacity, density and compressibility of water changes as water supercools. They found water can reach -48°C before it reaches its theoretical maximum crystallisation rate and must freeze.
Their model also showed that as water approaches -48°C, there is a sharp increase in the proportion of water molecules attached to four others to form tetrahedrons – the researchers have dubbed this intermediate ice.
“The water is transforming into something else, and this something else is very close to ice,” said Molinero, senior author of the study published in Nature.
As it freezes, water forms an amorphous ‘water glass’ with as many as a quarter of the molecules organised as intermediate ice or tiny ice crystals the study showed. As it approaches -48°C there is an unusual decrease in density and increase in heat capacity – which goes up rather than down. The water also gets easier to compress as it gets colder, and these unusual thermodynamics coincide with liquid water changing to the tetrahedral structure.
The researchers say this structural change explains the mystery of what determines the temperature at which water is going to freeze. They believe their work may have important implications for atmospheric scientists studying global warming, who want to know at what temperature and rates water freezes and crystallises into ice.
“You need 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,” said Molinero. “This is important for predictions of global climate.”
