At what temperature does water have to freeze?

At what temperature does water have to freeze? Not necessarily at zero degrees Celsius, say scientists.

Water can exist in liquid form well below this threshold, notably in a so-called ‘supercooled’ state. Scientists have found that rather than becoming ice once it’s cooled to zero degrees Celsius, supercooled liquid water can hold out until it reaches -48 degrees Celsius.

According to the study, if you want to form ice from liquid water, you need a ‘seed’ of ice from the liquid – a crystal that becomes the nucleus around which other crystals form. But in very pure water, which has no contaminants or particles around which the critical nucleus can form, this can be difficult to achieve because of the unusual thermodynamics of H20.

The findings suggest this structural change from liquid to an ‘intermediate ice’ explains the mystery of “what determines the temperature at which water is going to freeze”, said chemist Valeria Molinero from the University of Utah and senior author of the study published today in Nature. “This intermediate ice has a structure between the full structure of ice and the structure of the liquid. We’re solving a very old puzzle of what is going on in deeply supercooled water.”

A unique 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 said 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.

“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,” said Molinero.

Water’s density changes with temperature, and it is most dense at 3.8 degrees Celsius. 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 zero degrees and it still remains a liquid,” said Molinero.

No questions asked

Until now, supercooled water has been measured right down to around -41 degrees Celsius, although scientists have long suspected that the temperature at which it unconditionally has to freeze is somewhat lower. They have been unable to find out for sure because ice crystallises so fast at this temperature that it is impossible to measure accurately the properties of the remaining liquid.

Molinero and her colleague Emily Moore from the University of Utah used computer modelling to simulate the behaviour of supercooled water at the microscopic level. Their programme mimicked what would happen when 32,768 water molecules were cooled, factoring in the heat capacity of water, its density and compressibility. After thousands of computer hours, the answer came back. The temperature at which water absolutely must freeze, no questions asked, is -48 degrees Celsius.

Fodder for atmospheric scientists

When water approaches this temperature, it becomes less dense and becomes easier to compress and its structure changes. As a result, each molecule links up loosely with four others to form tetrahedron, or pyramid-like, shapes. The ‘intermediate ice’ is halfway between the structure of the liquid and halfway between the full structure of ice.

The research is more than an exercise in scientific curiosity. Atmospheric scientists delving into global warming need to know temperatures and rates at which water freezes and crystallises into ice. Water as chill as -40 degrees Celsius has been found in clouds. “You need that to predict how much water in the atmosphere is in the liquid state or crystal state,” Molinero said. “This is important for predictions of global climate.”