At first thought, it appears unremarkable that bodies that water frozen from the optimal down; after all, why wouldn"t they? certain the surface layer is closest come the cold air and also should frozen in location first?

In fact, almost every fluid freezes bottom-to-top, and also it"s only thanks come a specific quirk that chemistry the water bucks this trend.

You are watching: Why does a lake freeze from the top down instead of from the bottom up?

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Alexey Trofimov takes advantage of the unique chemical properties of water throughout his 500-mile trip across Siberian Lake Baikal.

Water at 4 °C

In general, a diminish in the temperature of a substance represents a lose in typical molecular kinetic energy.Essentially, cooler molecules "move" an ext slowly and the volume these molecules accounting is less than the of a hotter, much more energetic fluid. It complies with that the density (mass per unit volume) of a liquid increases together it cools and its constituent molecules occupy less space.

Water behaves no differently - that is, until it will 4 °C (39.2 °F).

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

Below 4 °C, the supposed trend reverses, and the cooling water becomes less thick with diminish temperature.This produces the an outcome that ice cream is less dense than water at day-to-day temperatures, which explains why ice cubes float in water and lakes "freeze over".It is likewise the reason that a can of soda bursts once left in the freezer; the water component of the drink increases in volume, compressing trapped air and pressurising the can past breaking point.

Hydrogen Bonding In A Lattice

To define this distinct characteristic, let"s think about the chemistry of water nearing the solid-liquid boundary.Although the specific structure of liquid hydrogen dioxide is unknown, us do know that water in ~ 4 °C comprises water molecules interacting by transient intermolecular hydrogen bonds.These attractive hydrogen bonds do water fairly dense for a pure (unimolecular) fluid - oil and also other straightforward organic substances have tendency to to rise on water.

However, together the water begins to shift from the fluid to the heavy phase, a type of molecular reshuffling occurs.Previously totally free molecules experiencing short-lived hydrogen bonding end up being fixed in a an ext energetically favourable crystalline lattice structure in i beg your pardon there room 4 teams hydrogen-bonded to each oxygen atom.This so-called open lattice is tetrahedral in shape and also occupies a greater volume every molecule 보다 water.

Ice, Protector of Life

Returning to our conversation of lakes, we deserve to now comprehensively describe why they freeze top-down.Our original intuition was the the surface layer must be coldest, and also this in itself is no incorrect. This is the factor that the very uppermost class of water molecule freezes first.However, in most liquids, this frozen crust would sink as, or even before, it develops - ~ all, hot fluids rise so cold liquids need to conversely sink - and layers of solid would construct from the upwards native the bottom the the liquid.In ice, the lighter late of ice remains atop the liquid, merely thickening as temperatures fall. This crust insulates the water beneath and also prevents big bodies that water freezing also in arctic temperatures.

If ice were come behave as various other liquids do, the consequences to life on planet would be profound.Annually, midlatitude bodies of water would become uninhabitable.Shallow bodies of water would certainly freeze solid and arctic seas would end up being supercooled by a slurry that ice.Billions of years of development would manifest an extremely differently: fish and other aquatic organisms not specialised because that sub-zero, icy conditions - in this dimension, most of them - would certainly not survive.The majority of naval life, at least as we recognize it, would not have the ability to stray much from the equator. Chances are the wouldn"t even exist.

This particular version of life on earth would be unrecognisable.

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Although the indestructible Tardigrade (which has actually been uncovered alive and well in arctic ice, volcanic soil and also the radiation-heavy vacuum of space) would most likely manage...

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References:

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