At initially believed, it appears unremarkable that bodies of water freeze from the optimal down; after all, why wouldn"t they? Surely the surface layer is closest to the cold air and should freeze in location first?

In reality, virtually every liquid freezes bottom-to-optimal, and also it"s just many thanks to a details quirk of chemisattempt that water bucks this trfinish.

You are watching: Why does water freeze from the top down

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Alexey Trofimov takes benefit of the unique chemical properties of water in the time of his 500-mile expedition across Siberian Lake Baikal.

Water At 4 °C

In basic, a decrease in the temperature of a substance represents a loss in average molecular kinetic energy.Basically, chillier molecules "move" more progressively and also the volume these molecules occupy is less than that of a hotter, more energetic liquid. It follows that the density (mass per unit volume) of a liquid increases as it cools and its constituent molecules occupy much less space.

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

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

Below 4 °C, the intended trfinish reverses, and the cooling water becomes less dense via decreasing temperature.This produces the outcome that ice is less thick than water at day-to-day temperatures, which describes why ice cubes float in water and also lakes "freeze over".It is likewise the reason that a can of soda bursts as soon as left in the freezer; the water component of the drink increases in volume, compressing trapped air and also pressuclimbing the deserve to beyond 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 exact framework of liquid hydrogen dioxide is unknown, we perform understand that water at 4 °C comprises water molecules connecting by transient intermolecular hydrogen bonds.These attrenergetic hydrogen bonds make water relatively dense for a pure (unimolecular) liquid - oil and also various other straightforward organic substances tfinish to float on water.

However before, as the water starts to shift from the liquid to the solid phase, a sort of molecular reshuffling occurs.Previously totally free molecules enduring short-lived hydrogen bonding end up being solved in a much more energetically favourable crystalline lattice framework in which tbelow are 4 groups hydrogen-bonded to each oxygen atom.This so-dubbed open lattice is tetrahedral in shape and occupies a better volume per molecule than water.

Ice, Protector Of Life

Returning to our discussion of lakes, we have the right to now comprehensively explain why they freeze top-down.Our original intuition was that the surface layer need to be coldest, and also this in itself is not incorrect. This is the reason that the extremely uppermany layer of water molecules freezes initially.However, in a lot of liquids, this frozen crust would sink as, or even before, it creates - after all, warm fluids climb so cold liquids need to conversely sink - and layers of solid would construct from the upwards from the bottom of the liquid.In ice, the lighter crust of ice continues to be atop the liquid, sindicate thickening as temperatures fall. This crust insulates the water beneath and also prevents big bodies of water freezing also in arctic temperatures.

If ice were to behave actually as other liquids carry out, the consequences to life on Earth would be prouncovered.Annually, midlatitude bodies of water would end up being uninhabitable.Shallow bodies of water would freeze solid and arctic oceans would certainly become supercooled by a slurry of ice.Billions of years of evolution would manifest very differently: fish and also various other aquatic organisms not specialised for sub-zero, icy conditions - in this dimension, the majority of of them - would not endure.The majority of marine life, at least as we know it, would certainly not have the ability to stray much from the equator. Chances are it wouldn"t also exist.

This certain version of life on Earth would be unrecognisable.

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

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References:

Klaus-Dieter KellerDiscussion of Tardigradeswww.bbc.co.ukwww.wikipedia.orgwww.britannica.comwww.physics.stackexreadjust.com