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. Because of the pressure, the water at the bottom of deep lakes can become cold without freezing to ice. Topics: density , freezing , frozen , ice , lake , lake ice , water , weather.
This process continues until the surface water cools below 4-degrees Celsius, at which point it becomes less dense, and eventually freezes. Remember, water is most dense at 4 degrees Celsius. It becomes less dense above and below this temperature. If water were most dense as a solid, lakes would freeze from the bottom up, eventually freezing solid. In that case, little or nothing would survive in the lake. Most lakes and ponds don't completely freeze because the ice and eventually snow on the surface acts to insulate the water below.
Our winters aren't long or cold enough to completely freeze most local water bodies. This process of lakes turning over is crtically important to the life in the lake. It is these turnover periods that infuse and distribute oxygen throughout the entire water column. Fish have several adaptions to survive a winter below the ice. First, they are cold blooded meaning their body temperature matches their environment. Colder temperatures mean a reduction in their metabolism.
This slows numerous metabolic processes, such as respiration, digestion, and activity level. Fish are often classified as coldwater, cool water, and warm water species. The warm water species will seek out the warmest water they can, which is often found at the bottom of the lake or pond. It is also the reason that a can of soda bursts when left in the freezer; the water component of the drink increases in volume, compressing trapped air and pressurising the can beyond breaking point.
To explain this unique characteristic, let's consider the chemistry of water nearing the solid-liquid boundary. These attractive hydrogen bonds make water relatively dense for a pure unimolecular liquid - oil and other simple organic substances tend to float on water.
However, as the water begins to transition from the liquid to the solid phase, a sort of molecular reshuffling occurs. 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.
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