Speculation: why water expands when it freezes

[chajadan]

I don't know the facts surrounding this issue, but I regularly hear that water is the only substance that expands when it freezes. (Personally I don't trust such a sweeping statement). But I also hear that we remain clueless as to why. It's the why I am speculating on.

 

I just imagine that it has something to do with the motion of the molecules as it relates to temperature, and the hydrogen bonds that I seem to gather are an attractive force between the molecules. A lower temperature slows the motions of the molecules, and perhaps the reduced speed inhibits the expression of the hydrogen bond attractions, and conversely, perhaps the hydrogen bonds become more expressed as the molecules speed up. I also assume there'd be some sort of density requirement to engage those forces, as a gas is too dispersed for the hydrogen forces to dominate the state of the system. Perhaps as the molecules move around more quickly, yet close enough (like in a liquid) to interact enough, the molecules bump more elbows at the party, constantly pulling towards one and then another and then another, until overall there is a contraction in the liquid -- all the shifting attractions keeping it tighter, than if it was prone to repel within itself. But as it cools and the molecules move more slowly, the party kind of dies down, the forces are more limited to "closer neighbors" (more accurately in idea, fewer neighbors), and so there is less "maintaining generalized contraction".

 

It's a sensical and appealing idea to me. If we were to seek out other molecules with known properties akin to those of the pervasive hydrogen bonds of water to see if they might expand on freezing, another example or examples could serve to support this idea.

 

--charlie

 

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So I was just looking into this idea, and the following wikipedia page is informative: https://en.wikipedia.org/wiki/Hydrogen_bond#Hydrogen_bonds_in_water

 

It does indicate that the number of hydrogen bonds varies with temperature -- more bonds at lower temperatures, and less as higher ones. That would seem contradictory to my idea: that ice has more bonds, but I think in fact more bonds implies stability, the hydrogen bonds are more likely to be fixed in an attraction, and not free to whirl around pulling in other party members.

 

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It may also be possible to experiment with the degree of expansion as water freezes with different solutes dissolved in. Perhaps the process can be teased into meaningful information.

 

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Edited March 4, 2014 by chajadan

[swansont]

I don't know the facts surrounding this issue, but I regularly hear that water is the only substance that expands when it freezes. (Personally I don't trust such a sweeping statement).

 

It probably is the case that some generalize this to the point that it's wrong. Water is one of the few substances that expands when it freezes.

 

I think if you Googled on Why does water expand when it freezes you'd find that "nobody knows" is incorrect by a large fraction of a million hits.

[chajadan]

Cool swansont! One of the dangers of thinking for one's self -- totally overlooking outstanding information ;-p

 

It's interesting to me, because, the way I read these descriptions of why water expands, they relate it to the spacious hexagonal structure that results from the hydrogen bonding. On the surface, this seems to sum it up as directly caused by something related yet other than the direct mechanism I suggested. It is cool to see that it is considered related to the hydrogen bonds =) =) And certainly it is how those bonds express themselves under the effect of temperature. I'm trying to see if my mechanism is just the other side of the same coin, rather than a distinct coin, from the architectural reason given. I read on that wiki page that hydrogen can participate in multiple hydrogen bonds at once. It seems frozen ice has significantly fewer of these multiple bonds. This would still relate to the "fewer rubbed elbows" idea.

[Endercreeper01]

When water expands, it forms a lattice. The atoms are further apart in the lattice then if they were all floating around, so the density increases.

[mooeypoo]

When water expands, it forms a lattice. The atoms are further apart in the lattice then if they were all floating around, so the density increases.

 

Actually, its volume increases. Its density actually decreases.

 

http://www.nyu.edu/pages/mathmol/textbook/info_water.html