Ice for Numb-ies:
In which I attempt to explain why ice floats and why that is a good thing

For a lot of us, chemistry is back in the hazy past and for at least some, it was hazy even then as eyes glazed over at the thought of covalent bonding and such. Time to rev it up again because there is something that is so very important about water chemistry that applies to our precious watershed and every body of water in the north. I will try to make it as unturbid as possible.

Water, remember, is made up of hydrogen and oxygen atoms, which are kind of like mutt and jeff. Hydrogen is very small, the smallest of all atoms. Its one electron zooms around its center single proton. Oxygen has a more complex structure and even though it is not a very large, in combination with hydrogen in the water molecule, it tends to hog the electrons from hydrogen so they are not shared equally. This means that the water molecule is more negative on one end and more positive where the hydrogens attach.

Now, since there are millions and millions of water molecules making up even a small amount of water, you can picture that the negative areas will attract the positive areas and vice versa, creating a kind of tug-of-war. This is very unlike any other liquid where electrons are equally shared, and is responsible for the defining characteristics of water. In the winter, in the north, one is the formation and character of ice, but there are other properties more noticeable in other seasons as well.

In the fall the water temperature starts to drop, and with it, if you could see what is happening at the molecular level, you would notice that the movement of the molecules slows d o w n. ( In a liquid, molecules are always moving and if the liquid is heated, they move faster; when it’s cooled, they slow). Along with this slowing comes an increase in the density of the liquid but with water, if you measure the density as water cools, it reaches a maximum density at 4 degrees C. or about 39 degrees F. This was the temperature of the lake water when I took it on December 18th. This was a lake whose entire water column had to be at 39 degrees because the denser colder water will always sink to the bottom. This was a lake just waiting for ice.

Getting back to chemistry for a quick moment, that slowly moving molecular water now is overpowered by the bonds between the adjacent molecules as they start to form a kind of grid held together by the opposite charges. At 39 degrees there are many molecules already in the grid formation but still enough of them fitting in between the grid. As the water cools further, these yet unattached molecules are forced out or added to the grid, which lessens the density. That water, less dense than that at 39 degrees, remains at the surface of the lake and is available for further cooling. At 32 degrees or 0 degrees C., all the molecules become fixed in the grid and solid ice forms. Ta-dah!! And since it is less dense even than 34 or 33 degree water, it floats! Underneath will always be heavier, denser liquid water, throughout the entire winter, no matter what happens on the surface.

So on my visit to the lake on Dec. 18, when I put in a thermometer and found the water to be 39 degrees, it was primed for freezing. If there were some windy days, that movement would prevent the grid from forming even as the water temperature dips below freezing, but ice might form at the windward edges. On a cold windless night, water will freeze first in protected areas and then in wider open ones, even freezing over completely in one night if the water is supercooled.

This year, it seems as if the lake has gone through a freeze/thaw cycle already. In reverse, when the air temperature warms, or the element of unseasonal rain occurs, the ice warms and the crystalline grid is disrupted, liquid water intrudes in the grid as ice melts and dense water is released. But not for long….

And now as snow covers the surface of the ice, you can picture life going on in very slow motion in the depths below, awaiting the release in the spring, thanks to this very unusual and unique chemistry property of water.


Many Thanks to member Amy Campbell for this article!