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Kdrama Kisses Everyone remembers the first time they were kissed by a kdrama! From the Seasonally Occupied Territories. Lining up the water molecules in this pattern takes up more space than having them jumbled randomly together as is the case in liquid water.
And because the same mass of molecules takes up more space when frozen, ice is less dense than liquid water. Close to freezing temperatures, the molecules in the liquid water begin to line up into the space-filling hexagonal structure. With further cooling and without mechanical mixing a stable, lighter layer of water forms at the surface.
As this layer cools to its freezing point, ice begins to form on the surface of the lake. In deep lakes, water pressure may also play a role.
The gravitational weight of all the water higher up in the lake presses down on the water deep in the lake. The pressure allows the water near the bottom of the lake to get cold without expanding and rising.
Previously free molecules experiencing short-lived hydrogen bonding become fixed in a more energetically favourable crystalline lattice structure in which there are 4 groups hydrogen-bonded to each oxygen atom. This so-called open lattice is tetrahedral in shape and occupies a greater volume per molecule than water. Returning to our discussion of lakes, we can now comprehensively explain why they freeze top-down. Our original intuition was that the surface layer must be coldest, and this in itself is not incorrect.
This is the reason that the very uppermost layer of water molecules freezes first. However, in most liquids, this frozen crust would sink as, or even before, it forms - after all, hot fluids rise so cold liquids must conversely sink - and layers of solid would build from the upwards from the bottom of the liquid. In ice, the lighter crust of ice remains atop the liquid, simply thickening as temperatures fall. This crust insulates the water beneath and prevents large bodies of water freezing even in arctic temperatures.
If ice were to behave as other liquids do, the consequences to life on Earth would be profound. Annually, midlatitude bodies of water would become uninhabitable. Shallow bodies of water would freeze solid and arctic oceans would become supercooled by a slurry of ice. Billions of years of evolution would manifest very differently: fish and other aquatic organisms not specialised for sub-zero, icy conditions - in this dimension, most of them - would not survive.
The majority of marine life, at least as we know it, would not be able to stray far from the equator. Chances are it wouldn't even exist. Although the indestructible Tardigrade which has been found alive and well in arctic ice, volcanic soil and the radiation-heavy vacuum of space would probably manage If you enjoyed this article and would like more, follow the everyday science blog for your daily dose of science.
Klaus-Dieter Keller Discussion of Tardigrades www.
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