|MadSci Network: Physics|
First letís consider pure liquids. (Solutions cooled below the point at which precipitation occurs are sometimes called supercooled but are perhaps more properly called supersaturated.) In principle any liquid can be supercooled. In general the more viscous a liquid and the weaker the intermolecular forces that make it a liquid the more easily it can be supercooled. Because of its low viscosity and strong hydrogen bonding water is actually rather difficult to supercool. Supercooling produces an unstable state where one phase of a material is at a temperature below that at which it would be in equilibrium with some other phase of the same material. Typically when a liquid is cooled to its normal freezing point some of it will change to the solid form and the temperature will stop decreasing as energy is removed and instead the rest of the liquid changes to a solid. But change takes time, so sometimesÖ if you donít cool it too slowly, or too rapidly, and there arenít any suspended solids (dust etc.) or too many scratches or dirt on the container and you donít shake it too much you can produce a container of liquid that is at a temperature below its freezing point. It may then stay in this state either very briefly or for quite some time before eventually solidifying rather abruptly. This solidification can also be triggered by the addition of a small amount of the solid form of the liquid (a seed crystal), shaking, which produces either bubbles or cavitation that act a nucleation sites, or even a speck of dust or scratching the wall of the container. You will sometimes hear that glass is a supercooled viscous liquid. For a good presentation of why this is probably not the best description for glass see Zanotto in the American Journal of Physics 66(5)392-395 1998. You will also sometimes see articles about supercooling a solution (one thing dissolved in another) below the point at which precipitation would normally occur. (The supersaturated solutions mentioned above) These exhibit behavior similar to a supercooled liquid except the liquid stays liquid even after all the solid has formed unless it becomes trapped in the crystals formed. For a good description of how to do this yourself see the page 80 of the October 2007 issue of Popular Science. You might still be able to bring this up from www.popsci.com under their How 2.0 or DIY sections.
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