Category: Chemistry Published: December 9, 2013
The freezing point of water drops below zero degrees Celsius as you apply pressure. Public Domain Image, source: Christopher S. Baird.
Yes, water can stay liquid below zero degrees Celsius. There are a few ways in which this can happen.
First of all, the phase of a material (whether it is gas, liquid, or solid) depends strongly on both its temperature and pressure. For most liquids, applying pressure raises the temperature at which the liquid freezes to solid. A solid is formed when the loose, meandering molecules of a liquid get slow enough and close enough to form stable bonds that pin them in place. When we apply pressure to a liquid, we force the molecules to get closer together. They can therefore form stable bonds and become a solid even if they have a higher temperature than the freezing point at standard pressure. Water is somewhat unique, though. Water molecules spread out when they are bonding into a solid crystalline structure. This spreading-out action leads ice to be less dense than liquid water, causing ice to float. This spreading-out action of the water molecules during freezing also means that applying pressure to water lowers the freezing point. If you apply enough pressure (making it hard for the water molecules to spread out into the solid structure), you can have liquid water several degrees below zero degrees Celsius.
Even if you don't apply pressure, you can still have liquid water at sub-zero temperatures using additives. Additives such as salt can interfere with the chemical bonding needed to form a solid and they therefore can lower water's freezing point. Salt is composed of strong sodium and chlorine ions. When dissolved in water, the water molecules tend to stick to the salt ions instead of to each other, and they therefore don't freeze as readily. As you add more salt to water, its freezing point continues to drop until the water reaches saturation and cannot hold any more salt. If you add enough salt, the freezing point of water can be dropped as low as -21 degrees Celsius. This fact means that water at -21 degrees Celsius can still remain liquid if enough salt is added. Instead of keeping liquid water from freezing, this powerful property of salt can also be used to turn ice back into water. Sprinkling salt on icy sidewalks lowers the freezing point of the ice below the ambient temperature and the ice melts. But sprinkling salt on icy walkways won't help if the ambient temperature is below -21 degrees Celsius. The effect of salt on water's freezing point also has profound effects on earth's oceans.
Even if you don't apply pressure and don't add anything to the water, you can still have liquid water at temperatures below zero degrees Celsius. In order for water to freeze to ice, it needs something to freeze onto to start the process. We call these starting points "nucleation centers". In most situations, a little bit of dust, impurity, or even little vibrations in the water provide nucleation centers for the water to freeze onto. But if your water is very pure and very still, there is nothing for the water molecules to crystallize onto. As a result, you can cool very pure water well below zero degrees Celsius without it freezing. Water in this condition is called "supercooled". At standard pressure, pure water can be supercooled to as low as about -40 degrees Celsius. Supercooled water is kept from freezing only by the lack of nucleation centers. Therefore, once nucleation centers are provided (which could be as simple as a vibration), the supercooled water quickly freezes. Freezing rain is a natural example of supercooled liquid water. Once freezing rain hits an object on earth's surface, that object provides nucleation centers, and the rain freezes to ice.
Topics: freezing, freezing point, freezing rain, ice, phase diagram, pressure, supercooled, temperature, water
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"Ice cold" just got even colder: By creating ice from tiny droplets only a few hundred molecules in size, researchers have pushed water's freezing point lower than ever before and changed what we know about how ice forms.
Knowing how and why water transforms into ice is essential for understanding a wide range of natural processes. Climate fluctuations, cloud dynamics and the water cycle are all influenced by water-ice transformations, as are animals that live in freezing conditions.
Wood frogs, for example, survive the winter on land by allowing their bodies to freeze. This allows them to come out of hibernation faster than species that spend the winter deep underwater without freezing. But ice crystals can rupture cell membranes, so animals that use this technique need to find a way to prevent ice from forming in their cells and tissues. A better understanding of how water freezes could lead to a better understanding of these extreme species.
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While the rule of thumb is that water freezes at 32 degrees Fahrenheit (0 degrees Celsius), water can actually stay liquid over a range of chilly temperatures under certain conditions. Until now, it was believed that this range stopped at minus 36 F (minus 38 C); any lower than that, and water must freeze. But in a study published Nov. 30 in the journal Nature Communications (opens in new tab), researchers managed to keep droplets of water in a liquid state at temperatures as low as minus 47.2 F (minus 44 C).
There were two keys to their breakthrough: very small droplets and a very soft surface. They began with droplets ranging from 150 nanometers, barely bigger than an influenza virus particle, to as small as 2 nanometers, a cluster of only 275 water molecules. This range of droplet sizes helped the researchers uncover the role of size in the transformation from water to ice.
"We covered all of these ranges so that we can understand at which condition ice is going to form — which temperature, which size of the droplets," study co-author Hadi Ghasemi, a mechanical engineering professor at the University of Houston, told Live Science. "And more importantly, we found that if the water droplets are covered with some soft materials, the freezing temperature can be suppressed to a really low temperature."
The soft material they used was octane, an oil that surrounded each droplet within the nanoscale pores of an anodized aluminum oxide membrane. That allowed the droplets to take on a more rounded shape with greater pressure, which the researchers say is essential for preventing ice formation at these low temperatures.
Because it's basically impossible to observe the freezing process at these small scales, the researchers used measures of electrical conductance — since ice is more conductive than water — and light emitted in the infrared spectrum to catch the exact moment and temperature at which the droplets transformed from water to ice.
They found that the smaller the droplet, the colder it had to be for ice to form — and for droplets that were 10 nanometers and smaller, the rate of ice formation dropped dramatically. In the smallest droplets they measured, ice didn't form until the water had reached a bone-chilling minus 44 C.
Does this mean that the microscopic droplets within clouds and biological cells can get even colder than we thought? "As a scientist, I would say we don't know yet," Ghasemi said.
But this discovery could mean big things for ice prevention on human-made materials, like those in aviation and energy systems, Ghasemi said. If water on soft surfaces takes longer to freeze, engineers could incorporate a mix of soft and hard materials into their designs to keep ice from building up on those surfaces.
"There are so many ways that you can use this knowledge to design the surfaces to avoid ice formation," Ghasemi said. "Once we have this fundamental understanding, that next step is just the engineering of these surfaces based on the soft materials."
Originally published on Live Science.
Ashley Hamer is a contributing writer for Live Science who has written about everything from space and quantum physics to health and psychology. She's the host of two podcasts: Curiosity Daily and Taboo Science. She has also written for the YouTube channels SciShow and It's Okay to Be Smart. With a bachelor's and master's degree in jazz saxophone from the University of North Texas, Ashley has an unconventional background that gives her science writing a unique perspective and an outsider's point of view.