View Full Version : ice rises
06-26-2000, 11:03 PM
Why does ice rise to the top of a lake, instead of sinking , as you would think it would because of density.
Its a good thing it does also; or lakes would flood every winter.
06-26-2000, 11:09 PM
It FLOATS because of its density.
Ice takes up more space than its equivelant amount of water -- remember is bursts the bottle when you freeze it -- thus it is less dense and floats!
06-26-2000, 11:22 PM
And if you're wondering why it's less dense than liquid water (it's a solid, after all, and most solids are denser than liquids), that's because it forms a crystal structure that ends up being less dense than the liquid.
Imagine that in the crystal, the atoms have to go into specific locations, which may leave extra space as opposed to the liquid.
I don't know how common this is; my guess is that there's at least a few compounds (probably with similarly simple molecules).
I'll let a chemist correct me if I'm wrong on this.
I'm not a chemist. Heck, I'm not even Australian.
06-27-2000, 12:48 AM
Water's behavior of getting less dense and larger when it freezes is very unusual. When liquid, the atoms in water can nestle together, but they spread apart and form an orderly crystalline structure when water freezes.
We'd be in a lot of trouble if water behaved like most substances and got denser when it solidified. Instead of the top of a lake freezing over, the ice would sink. Eventually, the lake would be a solid hunk of ice.
06-27-2000, 02:58 AM
Another example of a material that expands when it freezes is antimony (Sb). I have no idea if it has a diamond-type crystalline structure, like ice. It's useful for making type, because you don't want the metal to lose its shape to contraction as it's solidifying in the forms.
06-27-2000, 10:15 AM
Water partipates in something known as "hydrogen bonding." Water molecules are highly polar, meaning that they have a distinct "partially positive" end and a corresponding "partially negative" end. In the liquid phase, hydrogen bonding, which is a weak interaction betwene the positive "hydrogen" parts of one molecule and the negative "oxygen" parts of others, pulls the molecules in fairly close. However, this is a weak interaction, not really a formal "bond" so the molecules still have mobility. In its crystal structure, ice molecules have to form rigid bonds, locking in place with the molecules around it. This causes an increase in average molecular distance, which causes a decrease in density.
Let me make a thought-experiment analogy. Imagine a room full of people. They are all allowed to move around the room, so long as they maintain contact with two other people. They only need to touch two other people, but are free to roam within the group of people otherwise as they wish. Now imagine everyone stops moving and locks hands with two other people. In order to keep the whole structure of people standing still, you have to extend your arms and lock your elbows, to prevent any motion. In the former case, you can see how the people will take up less space than in the latter, since there is no requirement on rigid arm locking, merely contact. It's analagous with liquid water and ice.
I don't have a source handy, but other polar substances where hydrogen bonding particpates to a reasonable extent may also have solids that float on the liquid phase. This is a small group of molecules, probably just Ammonia and Hydrogen Floride, but if anyone has a MERCK or a CRC to check, you could look up and see if these molecules also behave liek water in this repect. I have a funny feeling they just might.
I have no idea about antimony, however.
06-27-2000, 10:24 AM
I am so glad you answered this one, Jayron. I had this covered in Chem class, and altho it made total sense at the time, I wasn't sure I could remember it well enough to exlain it.
06-28-2000, 12:00 PM
On a related note: Why does water have its maximum density at four degrees Celcius? Why does it expand if colder than 4C?
06-28-2000, 12:18 PM
The reason why the max density is a 4C is because below this temp the bonds are starting to from but have on yet solidfied. To go with the ablove anaolgy people are starting to straighten their arm but they have not yet locked them.
06-28-2000, 02:29 PM
When I was taking Organic Chemistry, my professor told us that liquid water has a "flickering crystalline structure." That is, on a very small scale, maybe 50 or 100 molecules will spontaneously align themselves into a crystal lattice similar to ice. These structures last a few milliseconds, and then the molecules return to their previous random motion.
He never related this property to the density of liquid water, but it seems like it would have some effect. Unfortunately, I don't know enough about chemistry to say for sure. Maybe one of our other posters can clarify.
06-28-2000, 02:41 PM
As far as I know, this is not necesarrily a property of merely liquid water, but of many liquids. It's important to remember that temperature is a measure of average molecular kinetic energy among teh entire group of molecules. There was a guy called "Boltzmann" who did some theorizing about probablitlies and energy densities and such. Boltzmann curves and the Boltzmann constant are named after him. Anywho, his idea was that we observe energy dependant traits of a substance (like its temperature) that is just an average; that many molecules exist at different energies. It is entirely possible that in a sample of liquid water at, say, 25 degrees celcius there are individual molecules of water moving at such a speed as to have a temperature of below 0 degrees C, and thus would, until something ran into them and sped them up, were able to form conglomerations of a few hundred molecules in "solid-like" form. However, this could be said of ANY liquid, and not just water; though water's participation in Hydrogen bonding may cause it to happen more in water.
06-28-2000, 05:48 PM
jayron 32 said
However, this could be said of ANY liquid, and not just water; though water's participation in Hydrogen bonding may cause it to happen more in water.Yes, this property of water was brought up within the context of a lecture on hydrogen bonding. The prof said that the flickering crystalline structure of liquid water is stronger than that of other liquids due to the H bonds. He suggested that this was one of the reasons why water was so hard when you jumped into it from a significant height. All the tiny, transient "crystals" in the water made it more solid.
Thanks for confirming this.
06-28-2000, 11:37 PM
suggested that this was one of the reasons why water was so hard when you jumped into it from a significant
Well, in the first case he was mostly right. Here, however, your professor is mostly talking out of his ass. The reason water hurts so much is that liquids and solids do not have a significant difference in density, and thus you are hitting a LOT of stuff at a great speed. Sure, liquid water is slightly more dense than ice, but only slightly. The density difference between liquids and solids is within about an order of magnitude (very close to 1, or almost no difference) in many cases. The density difference between a gas and a liquid or a solid is close to 1000 times less dense. Thus, you are moving through a fluid that has, say, a certain density, and then you hit one that is roughly 1000 times more dense. That is why it hurts. You could smack into a lake of gasoline, which has no inherent hydrogen bonding of any sort, and it would hurt just as much.
06-28-2000, 11:58 PM
Gallium expands upon freezing. It also does this at just slightly above room temperature, so that, unlike M&M's, it will melt in your hand.
06-29-2000, 12:02 AM
Is there a liquid that Doesn't freeze?
To the OP: You see when water freezes it forms a web like lattice where air gets trapped within. This is an easy way to describe why ice floats atop the water. As you are correct in your assumption that ice is less dense at the molecular level than water.
I think if you get cold enough any liquid will freeze up.
06-29-2000, 02:04 AM
i don't think helium (or at least a certain isotope thereof) freezes. it remains a fluid no matter how close to absolute zero we have gotten it, doesn't it? that's how we can get weird quantum vortices and whatnot. although i might be wrong...
06-29-2000, 09:50 AM
To the OP: You see when water freezes it forms a web like lattice where air gets trapped within. This
is an easy way to describe why ice floats atop the water. As you are correct in your assumption that
ice is less dense at the molecular level than water.
This is blatantly and unequivocally wrong. Ice does NOT flot AT ALL because of trapped air, it floats because it is less dense than water due to molecular concerns. Any theoretical "trapped air" (of whose existence I am highly skeptical in the first place) plays no bearing in this whatsoever.
i don't think helium (or at least a certain isotope thereof) freezes. it remains a fluid no matter how close to absolute
zero we have gotten it, doesn't it? that's how we can get weird quantum vortices and whatnot. although i might be
Solid helium does not exist, but helium does have two liquid "phases." The colder phase, rather than being more solidlike, is actually almost gas like, with the density of a liquid and the fluid mechanics of a gas. It's called "Superfluid Liquid II Helium" and is very weird stuff. And AFAIK, there is no such thing as "solid" helium. There is, however, solid Hydrogen, which appears to have metallic properties. Very interesting, if you ask me.
jayron 32 said: Ice does NOT flot AT ALL because of trapped air, it floats because it is less dense than water due to molecular concerns. Any theoretical "trapped air" (of whose existence I am highly skeptical in the first place) plays no bearing in this whatsoever.
I certainly agree that trapped air has no bearing at all on ice floating; however, why are you "highly skeptical" of its existance? I thought that air dissolved in water will stay trapped in the ice, which is why ice cubes are a milky color rather than crystal clear. (Unless you make your cubes out of recently boiled water.) I don't think, though, that the air is trapped in a "web like lattice" like Silo proposes. More likely, it's coming out of suspension in little bubbles. Care to enlighten me, jayron?
This is blatantly and unequivocally wrong. Ice does NOT flot AT ALL because of trapped air, it floats because it is less dense than water due to molecular concerns.
Whoops! Oh well everyone makes mistakes sometimes.
06-29-2000, 09:01 PM
Yes, it is possible to solidify helium, it's just really, really difficult. IIRC, it takes something like .001 Kelvin and 1000 or so atmospheres of pressure-- it's never been done at standard pressure.
The temperature of the phase transition from liquid to superliquid depends strongly on the isotope of the helium (He-3 has to get a lot colder); this is probably what jb_farley is thinking of with the isotopes.
06-30-2000, 12:13 AM
Care to enlighten me, jayron?
Sorry, I was asuming that Silo was refering to some system whereby molecules of air (nitrogen, oxygen, or argon) are trapped between the molecules of water in the ice latice, and I was highly skeptical of their existence. The translucence of ice is do to visible bubbles of air, which are decidedly macromolecular in nature (uh, you can see'em) and also aren't trapped by ice's crystal latice, they are extracrystalary also (is that a word?) Sorry about the confusion.
And chronos, I defer to you on solid helium. I don't know it all, and appatently you know more in this case.
Ahh. OK, Jayron. I interpreted Silo's comment the same as you did. I just got confused when you said you were skeptical of the existance of any "theoretical" trapped air. Thanks for the clarification.
06-30-2000, 11:56 AM
An interesting aside to this conversation, is why a lake freezes from top to bottom.
As water cools it becomes more dense, thus the colder water sinks to the bottom and the less dense warmer water rises to the top. This convection continues until the water is a homogenous 4 degrees C. Below this temp the density of water begins to decrease with decrease in temperature so that it no longer sinks. On further cooling the water begins to freeze at the surface. The ice layer doesn't sink because it's less dense than the liquid and acts as a thermal isolator for the water below.
Ice fishermen rejoice!
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