At an initial thought, it seems unremarkable that bodies the water freeze from the peak down; after ~ all, why wouldn"t they? for sure the surface ar layer is closest come the cold air and also should freeze in place first?

In fact, nearly every liquid freezes bottom-to-top, and it"s only thanks come a particular quirk that chemistry that water bucks this trend.

You are watching: Why do lakes freeze from the top down

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Alexey Trofimov takes benefit of the one-of-a-kind chemical nature of water throughout his 500-mile trip throughout Siberian Lake Baikal.

Water at 4 °C

In general, a diminish in the temperature that a problem represents a lose in average molecular kinetic energy.Essentially, chillier molecules "move" much more slowly and also the volume these molecules accounting is less than the of a hotter, much more energetic fluid. It adheres to that the density (mass per unit volume) that a liquid increases together it cools and also its constituent molecules occupy much less space.

Water behaves no in different way - that is, till it reaches 4 °C (39.2 °F).

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Very cold water is less thick than water at certain greater temperatures, and also ice less dense still.

Below 4 °C, the supposed trend reverses, and also the cooling water i do not care less thick with decreasing temperature.This to produce the result that ice cream is less dense than water at daily temperatures, which explains why ice cream cubes to rise in water and lakes "freeze over".It is likewise the factor that a have the right to of soda bursts as soon as left in the freezer; the water ingredient of the drink increases in volume, compressing trapped air and pressurising the can past breaking point.

Hydrogen Bonding In A Lattice

To describe this distinct characteristic, let"s think about the chemistry of water nearing the solid-liquid boundary.Although the precise structure of liquid hydrogen dioxide is unknown, us do know that water at 4 °C comprises water molecules connecting by transient intermolecular hydrogen bonds.These attractive hydrogen bonds do water relatively dense because that a pure (unimolecular) liquid - oil and also other basic organic substances have tendency to float on water.

However, together the water begins to transition from the liquid to the hard phase, a sort of molecular reshuffling occurs.Previously free molecules experiencing short-lived hydrogen bonding come to be fixed in a much more energetically favourable crystalline lattice structure in i beg your pardon there room 4 groups hydrogen-bonded to every oxygen atom.This so-called open lattice is tetrahedral in shape and also occupies a higher volume every molecule 보다 water.

Ice, Protector of Life

Returning come our discussion of lakes, we deserve to now comprehensively explain why they frozen top-down.Our original intuition was the the surface ar layer should be coldest, and also this in itself is no incorrect. This is the factor that the an extremely uppermost layer of water molecule freezes first.However, in most liquids, this frozen crust would sink as, or also before, it develops - after ~ all, hot fluids climb so cold liquids should conversely sink - and also layers of heavy would build from the upwards native the bottom the the liquid.In ice, the lighter tardy of ice stays atop the liquid, merely thickening together temperatures fall. This crust insulates the water beneath and also prevents big bodies of water freezing even in arctic temperatures.

If ice cream were come behave as other liquids do, the results to life on earth would it is in profound.Annually, midlatitude body of water would end up being uninhabitable.Shallow bodies of water would freeze solid and arctic oceans would end up being supercooled through a slurry of ice.Billions of years of development would manifest really differently: fish and also other aquatic organisms not specialised because that sub-zero, icy conditions - in this dimension, many of lock - would not survive.The bulk of maritime life, at least as we recognize it, would certainly not have the ability to stray far from the equator. Chances are the wouldn"t also exist.

This particular version that life on earth would be unrecognisable.

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Although the indestructible Tardigrade (which has been found alive and also well in arctic ice, volcano soil and also the radiation-heavy vacuum that space) would probably manage...

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References:

Klaus-Dieter KellerDiscussion the Tardigradeswww.bbc.co.ukwww.wikipedia.orgwww.britannica.comwww.physics.stackexchange.com