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.
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Topics: freezing, freezing point, freezing rain, ice, phase diagram, pressure, supercooled, temperature, water
