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Post by Babu on Mar 17, 2021 8:42:42 GMT -5
Water has a maximum density at 4'C, which means ice close to freezing floats on top of warmer water. I've heard a lot of scientists explaining that this is the reason fish can survive, because the ice will form on top of the water and isolate the rest of the water, and that if 0'C water, and ice, was more dense than 1'C water, ice would form at the surface and then sink to the bottom making less and less room for the fish to swim in until all, or nearly all of the lake is frozen and all the fishes are just lying on the surface of the lake.
But I'd argue that the vast majority of the world's lakes and seas, at least outside of the arctic and other extremely cold region, a linear water density would mean lakes would form zero ice at all. The reason is that currently, lakes can form ice on the surface despite the water underneath being 4'C. If the water density decreased linearly however, water near the surface that would cool down would sink (before freezing), and be replaced by warmer water from underneath. There'd be a constant convective recycling of surface water until all of the lake was exactly 0'C, and then ice would start to titrate from the surface and fall down towards the bottom of the lake kinda like snow. In extremely cold places, lakes would be able to be completely frozen over, but in most places, lakes would never be able to cool down to the point where all of the lake was 0'C, and so they would never form ice at all.
This is my theory, and I'm curious to see if you think it's correct or not
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Post by Speagles84 on Mar 17, 2021 13:28:57 GMT -5
Liquid water on a molecular level is fluid and individual molecules at low temperatures can fill "voids" easy. When ice is formed, as with all solids, a lattice geometric pattern develops. When an incalculable amount of water molecules begin to freeze (lake), the pattern of how it freezes would vary on so many variables. Personally, the water temperature profile of a lake is the strongest variable. The water exposed to cold air at the surface is likely to be significantly cooler than that underneath (during winter). Since the ice molecules form relatively quickly and are only 90% of the maximum density, they continue to float. They are so small you can't see them, hence water temperatures reading at 32F/0C exactly - this is likely water that is 50% ice molecules and 50% 33F water molecules. These molecules continue to float in the water whilst the others sink slightly. Also the rate the molecules would sink is SO slow, its not really likely they would reach "deep" depths as to keep the surface ice free regardless.  |
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Post by Babu on Mar 17, 2021 13:37:47 GMT -5
Liquid water on a molecular level is fluid and individual molecules at low temperatures can fill "voids" easy. When ice is formed, as with all solids, a lattice geometric pattern develops. When an incalculable amount of water molecules begin to freeze (lake), the pattern of how it freezes would vary on so many variables. Personally, the water temperature profile of a lake is the strongest variable. The water exposed to cold air at the surface is likely to be significantly cooler than that underneath (during winter). Since the ice molecules form relatively quickly and are only 90% of the maximum density, they continue to float. They are so small you can't see them, hence water temperatures reading at 32F/0C exactly - this is likely water that is 50% ice molecules and 50% 33F water molecules. These molecules continue to float in the water whilst the others sink slightly. Also the rate the molecules would sink is SO slow, its not really likely they would reach "deep" depths as to keep the surface ice free regardless. What does that say about an alternate reality where water density increases linearly with temperature? |
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Post by Speagles84 on Mar 17, 2021 13:39:33 GMT -5
Liquid water on a molecular level is fluid and individual molecules at low temperatures can fill "voids" easy. When ice is formed, as with all solids, a lattice geometric pattern develops. When an incalculable amount of water molecules begin to freeze (lake), the pattern of how it freezes would vary on so many variables. Personally, the water temperature profile of a lake is the strongest variable. The water exposed to cold air at the surface is likely to be significantly cooler than that underneath (during winter). Since the ice molecules form relatively quickly and are only 90% of the maximum density, they continue to float. They are so small you can't see them, hence water temperatures reading at 32F/0C exactly - this is likely water that is 50% ice molecules and 50% 33F water molecules. These molecules continue to float in the water whilst the others sink slightly. Also the rate the molecules would sink is SO slow, its not really likely they would reach "deep" depths as to keep the surface ice free regardless. What does that say about an alternate reality where water density increases linearly with temperature? I would assume the lakes would freeze on the surface, ice would sink, and increase from the bottom up. Assuming no oddities in the chemistry in that universe |
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Post by Babu on Mar 17, 2021 14:04:33 GMT -5
What does that say about an alternate reality where water density increases linearly with temperature? I would assume the lakes would freeze on the surface, ice would sink, and increase from the bottom up. Assuming no oddities in the chemistry in that universe So you don't think convection flow in the water would prevent ice from forming until all of the water in the lake was close to 0'C. I'd love to see an experiment trying this using other liquids with more linear temperature expansion. Mercury comes to mind but is problematic since you can't see through it. |
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Post by urania93 on Mar 17, 2021 15:42:19 GMT -5
You are hypothesising a world where water molecules don't experience hydrogen bonding, chemically (and also physically) speaking it would make quite a lot of difference... However, the chemical reason why ice is less dense than liquid water lies in the fact that hydrogen bonds favour the formation of a crystal structure in which the water molecules are arranged leaving more empty space than in the liquid. This happens only for molecules in which hydrogen bond interactions are possible because, unlike the other intermolecular forces, it is quite directional and so specific geometries can be much more energetically favoured than others. On the other hand, the majority of liquids only show non-directional intermolecular interactions called Van der Waals forces, and thus when turning solid tend to favour very closed-packed crystal phases. There are quite a lot of apolar organic solvents belonging to this category, such as as hexane or benzene, which are completely transparent.  Returning to the hypothetical lake example with sinking ice, in theory I think that it should work as you theorize in the first post, the cooling water at the surface would continue to sink and to be replaced by the warmer water below it until the lake approaches 0°C, and then the surface would start to crystallize and sink. I have the suspect that comparing a system like this one, which always mixes, with a stratified system like real world lakes in winter, could hide some tricks related to the kinetics of the process. However, it is quite evident that that hypothetical lake would need to release more heat toward the cold air before completely freezing than a real world lake where only the top layer freezes. Anyway, all this stuff is reminding me of some stuff we studied back in the high school about the seasonality of lakes mixing, I don't remember much of it but this is when wikipedia comes to help... [1] |
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Post by Crunch41 on Mar 17, 2021 23:20:18 GMT -5
I agree with the other posts that the lake would freeze from the bottom up and require more time to freeze. Urania added a picture of solid Benzene. If you wanted to try this in real life, get some glacial acetic acid. It freezes at 16-17C, slightly below room temperature.
The solid is denser than the liquid so the container can (and does) freeze during shipping and storage but still can be used when it thaws out. The liquid is clear but the ice can have some cloudiness to it like solid frozen water. If the annual average temperature was above freezing, the ground at the depth of the lake would also be above freezing, so eventually some of the ice layer at the bottom would melt. Since the liquid is less dense than the solid, any thaw would cause an increase in volume, so it might form a water layer like an underground lake, or it might generate enough force to break the ice layer and mix with the liquid above the ice.
 If the annual average temperature was below freezing, eventually the lake would freeze solid I think. Underground lakes could still be possible but I don't know how they would form. |
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