HOW COME? When water doesn't freeze
In the winter, I store bottled water and other beverages in single-serving containers on my unheated enclosed porch. Even when the temperature stays well below freezing, the bottles have never frozen, writes Vicky Ayers. However, I have noticed something even stranger: When I bring a bottle into the warm house, it may suddenly freeze, turning to slush. Once, I poured a bottle of water over ice cubes in an insulated container and it turned to hard-packed snow. My co-workers claim I am living in a vortex. What the heck is happening?Hey, don't blame the house. You might experience the same instant icing effect in a bottle of water left in a frigid car overnight. It turns out that liquids need more than freezing temperatures to make the switch from sloshy to slushy.
Down to temperatures well below freezing, a liquid still requires a nucleus -- say, a tiny bit of mineral dust -- around which an ice crystal can form. As crystals form and connect, the liquid freezes. But without such nuclei, a liquid can remain crystal-free as the temperature drops -- and drops.
We all "know" that water's freezing point is 32 degrees F. But in fact, if the water is pure enough, it can "supercool" before it actually freezes. With no nucleation sites -- say, in a smooth plastic bottle of water, purified by reverse osmosis -- water can supercool to a temperature below 32 F.
But when you move, shake, or open the very cold bottle, the tenuous structure is disturbed, gas bubbles may come out of solution, dust may drop from the cap. And the water instantly turns to slush, as ice spreads through the container. Pour over ice, and the freezing accelerates.
But what if the water is left alone? In 2011, University of Utah researchers proved that water's bottom-line freezing temperature is -55 F, or 87 degrees colder than the relatively balmy 32 F. Pure water finally turns to ice, each H2O molecule bonding to four others, forming icy tetrahedrons. This "intermediate ice" kick-starts the formation of icy crystals throughout the water, which then freezes solid.
Supercooled water is also behind the freezing rain and ice storms of winter. As raindrops fall, passing through layers of very cold air, they can become supercooled. (Inside clouds, some liquid droplets survive even when the temperature drops as low as -40 degrees F.)
But when the frigid drops hit surfaces, from tree limbs to power lines to sidewalks and roads, they freeze instantly. The result: An icy, slippery glaze on everything the supercooled rain touches.