See headline. Seems that most things contract in extreme cold, but water expands. Why?
It doesn’t expand. If it doesn’t freeze, cold water sinks to the bottom, just as you would expect.
Mod, headline should read “does” instead of did. I can’t figure out how to fix it. Please fix. Thanks
Water is at its most dense at 4 degrees Celsius (3.98 to be exact).
Before/after that it get less dense.
When water freezes, it crystallizes (this is what I was taught in Chem class). The crystallised form takes up more space than the amorphous liquid form, hence an expansion at that particular temperature point.
The question is: do other substances do the same thing when they freeze? In order for us to get data, we’d have to think of some substances that have very simple structure, like water (so would be likely to crystallize at all) and that we commonly encountered in both liquid and solid form so that we could compare them.
I can’t immediately think of any good candidates. Alcohol might be one, but alcohol freezes pretty low and I’m having difficulty finding out what the density of solid alcohol is. And even alcohol is a more complex molecule than water.
Metals are generally crystallised. So maybe some metals get less dense when they melt? I’m not sure where to find the data on that
Because when water freezes and solidifies it’s molecular structure turns to crystals and the crystals form a lattice (with air gaps between them). That ice crystal lattice is physically larger than the cold water it was before it froze. The air molecules trapped between the crystals are why it’s density is less and it floats.
duh … MORE dense, that should be, obviously
(does an Aspidistra get more dense when she posts before coffee?)
There are some other substances that expand when changing from liquid to solid state. Two that I remember off the top of my head are gallium and bismuth, both metallic elements. Gallium melts at slightly above room temperature, and you are warned to leave space for expansion in a container you pour liquid gallium into. It also may break a glass container when it expands upon freezing (as well as wetting glass which can be rather inconvenient).
Ice is less dense than water even when no air is present.
And there’s no room for air molecules within the molecular lattice itself. Air molecules are larger than water molecules. Air certainly gets trapped inside ice to varying degrees, making it even less dense overall, but that air is trapped in gaps and fissures, not within the crystalline structure itself.
The only metal I know that does that is Tin (one of the allotropes of tin).
Some other elements that does this are:
silicon, germanium and bismuth
It’s the hydrogen bonding that plays a big part.
Planets that have ammonia or methane atmospheres will have the solids in the bottom of the “seas” if cold enough. Ice also reflects sunlight thereby prolonging ice ages on earth - may not be so on other planets
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Yes. Normally a crystal structure is denser than a randomly oriented structure - you can fit more cubical boxes in a volume if you stack them regularly than if you just toss them in the container willy-nilly. But in water, the hydrogen bonding keeps water molecules oriented so that relatively large gaps exist in the crystal structure, making it less dense than a randomly oriented set of molecules (as if the boxes you were stacking had the magnets at each corner (north poles out, so that the boxes could only be regularly stacked in a way that had large gaps).
That’s because ice crystals begin to form. If you prevent that (raise the pressure?) the water density will continue to increase as the temperature drops.