4 ms·

Interestingly liquid water also has negative thermal expansion between 0 and 3.98°C. This means that natural convection ("heat rises") is reversed in cold water

Interestingly liquid water also has negative thermal expansion between 0 and 3.98°C. This means that natural convection ("heat rises") is reversed in cold water. https://en.m.wikipedia.org/wiki/Negative_thermal_expansion https://en.m.wikipedia.org/wiki/Negative_thermal_expansion

rtaylorgarlock 2y ago
I think about this way more often when I should. I'm prototyping a cooling system and currently using ice; designing tolerance for the cool reservoir to contract a little as ice warms and changes states has my brain hooked on the weird behaviors of water.
- BonoboIO 2y ago
  What are you cooling?
vishnugupta 2y ago
And for this reason ice floats on water as it’s one of those rare materials whose density decreases as it solidifies. Because ice floats and is an excellent insulator rivers and lakes don’t completely freeze up enabling aquatic life to continue to thrive beneath while the surface remains frozen.
- pyinstallwoes 2y ago
  So outer space is max frozen water?
- beltsazar 2y ago
  If ice didn't float, life as we know it wouldn't be possible anywhere in the universe. (It doesn't exclude life that doesn't require water, though—if that's possible.)
  - WarOnPrivacy 2y ago
    > If ice didn't float, life as we know it wouldn't be possible anywhere in the universe. I cheerfully propose this is evidence of a hacked universe. More specifically that there was a design flaw in the core structures and that water got tweaked as a band-aid - rather than rip the germ-code apart and rewrite it. subscript: my premise requires a fallible creator. the most interesting kind, imho
    - Shadowmist 2y ago
      water is OP
    - baq 2y ago
      if we live in a simulation, god must exist
      - HPsquared 2y ago
        What's the next level above hypervisor?
        baq 2y ago
        it's gods all the way up
    - seadan83 2y ago
      Evidence and a clue are very different. Just because a scientific hypothesis us incorrect, or there is no explanation, does not mean that alternatives must be right. Eg: disproving darwinism does not mean creationism is correct. Just that darwinism is flawed. You have to affirmatively prove alternative theories, can't just disprove something else and claim that makes you right. Yet more, creationism is not falsifiable, that's outside the scope of science. In the realm of faith, clues are taken as evidence, and this is where I quibble with you if we are talking science fact.
      - WarOnPrivacy 2y ago
- dev_tty01 2y ago
  Yes, ice floating is super important in our system. Another biggie is the surface tension effect. Surface tension [1] plays a big part in the capillary action that allows trees to transport water and nutrients from deep in the ground up to the top of their trunk and branches. Surface tension is important throughout nature and many industrial processes as well. Water is incredible. [1] https://www.fs.usda.gov/research/treesearch/38556 https://www.fs.usda.gov/research/treesearch/38556
  - vishnugupta 2y ago
    > Water is incredible. You could say that again! The more I observe and understand properties of water the more fascinating it gets. For example, the minutest of leakages can cause a havoc in a building. That’s why I get extremely nervous when I see damp walls. Or take flowing water. What looks like a gentle stream could easily sweep away a few tons of bus. Often times vehicle drivers misjudge this to drive through a shallow rain stream and end up in a whole lot of trouble.
v7n 2y ago
My father taught us this as a simple conjecture of sorts that somehow always stuck with me: "in a frozen lake the warmest layer of water is at the bottom and it's at 4°C". Now there might be a plethora of cases where this is not exactly true, as is often the case with simplified models, but this is how I would word it to my kids :)
- HPsquared 2y ago
  I wonder if that could be used to calibrate thermometers.
  - Iulioh 2y ago
    We already have the frozen and boiling temperature of water or any other liquid. I remind you that boiling water is 100c and not higher as the energy is used to change phase. (at 1atm and close to the sea level) You can use different materials for the temperature you are more interested in if you want more precision, we don't need to go under bodies of water to calibrate thermometers
    - steve_adams_86 2y ago
      I think they meant that hypothetically, presuming a case where we didn’t have access to better methods. The question might be rephrased as “would this be accurate and reliable enough to calibrate a thermometer if someone cared to try?”. I might be wrong, but it’s how I interpreted it.
- zeristor 2y ago
  As a child I used to main line OU lectures on Saturday mornings. One was on Ecology, and had about thermal inversion in a lake in the Lake District when the temperature of the lake fell to 4ºC and the thermal circulation changed, between above 4ºC and below. https://en.wikipedia.org/wiki/Lake_stratification#Definition https://en.wikipedia.org/wiki/Lake_stratification#Definition
- sandworm101 2y ago
  It is a simplified model. The obvious next question is why doesn't the entire lake freeze from the top down? Why does the ice not get progressively thicker every day? The next-level is to model the thermal insulation of the ice. Heat from beneath is insulated from the cold air above. With a constant supply of heat from below but a reduced heat transfer due to the ice/snow, along with reduced evaporation, it is theoretically possible for the depths of a lake to get warmer once frozen over. https://en.wikipedia.org/wiki/Lake_stratification https://en.wikipedia.org/wiki/Lake_stratification
Traubenfuchs 2y ago
I think you can put this in much more lay friendly words if you say: water has the highest density at 3.98°C.
- remuskaos 2y ago
  And vice versa! The more you try to compress water, the closer you "push" it towards 3.98 centigrade. That's why water stays liquid below a certain depth, for example in the sea. Without that fact, water would freeze at higher pressures and no life would have developed in the ocean depths.
  - Traubenfuchs 2y ago
    What does that mean for extreme environments like… a black hole? Water gets sucked in, experiences ultimate gravity and compression bringing it close to stopped time… at almost perfect 3.98?
    - remuskaos 2y ago
      Well, no, for more than one reason. First, for the water, time doesn't stop. Chances are it wouldn't even know it surpassed the event horizon (you know, if it could actually experience anything). For an external observer, it would crawl to a halt just outside the event horizon until it simply fades from vision. Then gravitational tidal forces aren't strictly a uniform compression, on the contrary. Since gravity increases by 1/r^2, the part of the water drop that's closest to the black hole would experience largest force. That tears everything apart, so the water would be more dispersed instead of compressed. While I haven't done the math (and don't know if I even still could), I'd assume that gravity would quickly overcome Van Der Waals forces that holds the water molecules together, so it'd probably closer to water vapour than liquid. And lastly, once it finally reaches the singularity, all bets are off. As far as I'm aware we have no idea what happens to matter in a singularity apart from contributing to its mass.
vanderZwan 2y ago
Does this negative thermal expansion continue for supercooled water as well?
andrewflnr 2y ago
I wonder if this is tied to the (alleged, highly dubious) effect where hot water freezes faster then cold water? I always figured if it was real, it had to be some momentum-based phenomenon. So if (a) hot water in a freezer sets up strong convection currents and (b) those currents are more useful for cooling the water than the "usual" reverse convection of cold water (by bringing warmer water to the surface where its less insulated, or at least not actively burying it) and (c) the regular convection is able to continue on momentum even when the water has cooled a lot, then it might conceivably accelerate cooling enough to win the race. Maybe in just enough of a patchwork, fragile way to match the mess of inconclusive tests about the whole phenomenon.

rtaylorgarlock 2y ago

I think about this way more often when I should. I'm prototyping a cooling system and currently using ice; designing tolerance for the cool reservoir to contract a little as ice warms and changes states has my brain hooked on the weird behaviors of water.

BonoboIO 2y ago
What are you cooling?

vishnugupta 2y ago

And for this reason ice floats on water as it’s one of those rare materials whose density decreases as it solidifies. Because ice floats and is an excellent insulator rivers and lakes don’t completely freeze up enabling aquatic life to continue to thrive beneath while the surface remains frozen.

pyinstallwoes 2y ago
So outer space is max frozen water?
beltsazar 2y ago
If ice didn't float, life as we know it wouldn't be possible anywhere in the universe. (It doesn't exclude life that doesn't require water, though—if that's possible.)
- WarOnPrivacy 2y ago
  > If ice didn't float, life as we know it wouldn't be possible anywhere in the universe. I cheerfully propose this is evidence of a hacked universe. More specifically that there was a design flaw in the core structures and that water got tweaked as a band-aid - rather than rip the germ-code apart and rewrite it. subscript: my premise requires a fallible creator. the most interesting kind, imho
  - Shadowmist 2y ago
    water is OP
  - baq 2y ago
    if we live in a simulation, god must exist
    - HPsquared 2y ago
      What's the next level above hypervisor?
      - baq 2y ago
        it's gods all the way up
  - seadan83 2y ago
    Evidence and a clue are very different. Just because a scientific hypothesis us incorrect, or there is no explanation, does not mean that alternatives must be right. Eg: disproving darwinism does not mean creationism is correct. Just that darwinism is flawed. You have to affirmatively prove alternative theories, can't just disprove something else and claim that makes you right. Yet more, creationism is not falsifiable, that's outside the scope of science. In the realm of faith, clues are taken as evidence, and this is where I quibble with you if we are talking science fact.
    - WarOnPrivacy 2y ago
dev_tty01 2y ago
Yes, ice floating is super important in our system. Another biggie is the surface tension effect. Surface tension [1] plays a big part in the capillary action that allows trees to transport water and nutrients from deep in the ground up to the top of their trunk and branches. Surface tension is important throughout nature and many industrial processes as well. Water is incredible. [1] https://www.fs.usda.gov/research/treesearch/38556 https://www.fs.usda.gov/research/treesearch/38556
- vishnugupta 2y ago
  > Water is incredible. You could say that again! The more I observe and understand properties of water the more fascinating it gets. For example, the minutest of leakages can cause a havoc in a building. That’s why I get extremely nervous when I see damp walls. Or take flowing water. What looks like a gentle stream could easily sweep away a few tons of bus. Often times vehicle drivers misjudge this to drive through a shallow rain stream and end up in a whole lot of trouble.

v7n 2y ago

My father taught us this as a simple conjecture of sorts that somehow always stuck with me: "in a frozen lake the warmest layer of water is at the bottom and it's at 4°C". Now there might be a plethora of cases where this is not exactly true, as is often the case with simplified models, but this is how I would word it to my kids :)

HPsquared 2y ago
I wonder if that could be used to calibrate thermometers.
- Iulioh 2y ago
  We already have the frozen and boiling temperature of water or any other liquid. I remind you that boiling water is 100c and not higher as the energy is used to change phase. (at 1atm and close to the sea level) You can use different materials for the temperature you are more interested in if you want more precision, we don't need to go under bodies of water to calibrate thermometers
  - steve_adams_86 2y ago
    I think they meant that hypothetically, presuming a case where we didn’t have access to better methods. The question might be rephrased as “would this be accurate and reliable enough to calibrate a thermometer if someone cared to try?”. I might be wrong, but it’s how I interpreted it.
zeristor 2y ago
As a child I used to main line OU lectures on Saturday mornings. One was on Ecology, and had about thermal inversion in a lake in the Lake District when the temperature of the lake fell to 4ºC and the thermal circulation changed, between above 4ºC and below. https://en.wikipedia.org/wiki/Lake_stratification#Definition https://en.wikipedia.org/wiki/Lake_stratification#Definition
sandworm101 2y ago
It is a simplified model. The obvious next question is why doesn't the entire lake freeze from the top down? Why does the ice not get progressively thicker every day? The next-level is to model the thermal insulation of the ice. Heat from beneath is insulated from the cold air above. With a constant supply of heat from below but a reduced heat transfer due to the ice/snow, along with reduced evaporation, it is theoretically possible for the depths of a lake to get warmer once frozen over. https://en.wikipedia.org/wiki/Lake_stratification https://en.wikipedia.org/wiki/Lake_stratification

Traubenfuchs 2y ago

I think you can put this in much more lay friendly words if you say: water has the highest density at 3.98°C.

remuskaos 2y ago
And vice versa! The more you try to compress water, the closer you "push" it towards 3.98 centigrade. That's why water stays liquid below a certain depth, for example in the sea. Without that fact, water would freeze at higher pressures and no life would have developed in the ocean depths.
- Traubenfuchs 2y ago
  What does that mean for extreme environments like… a black hole? Water gets sucked in, experiences ultimate gravity and compression bringing it close to stopped time… at almost perfect 3.98?
  - remuskaos 2y ago
    Well, no, for more than one reason. First, for the water, time doesn't stop. Chances are it wouldn't even know it surpassed the event horizon (you know, if it could actually experience anything). For an external observer, it would crawl to a halt just outside the event horizon until it simply fades from vision. Then gravitational tidal forces aren't strictly a uniform compression, on the contrary. Since gravity increases by 1/r^2, the part of the water drop that's closest to the black hole would experience largest force. That tears everything apart, so the water would be more dispersed instead of compressed. While I haven't done the math (and don't know if I even still could), I'd assume that gravity would quickly overcome Van Der Waals forces that holds the water molecules together, so it'd probably closer to water vapour than liquid. And lastly, once it finally reaches the singularity, all bets are off. As far as I'm aware we have no idea what happens to matter in a singularity apart from contributing to its mass.

vanderZwan 2y ago

Does this negative thermal expansion continue for supercooled water as well?

andrewflnr 2y ago

I wonder if this is tied to the (alleged, highly dubious) effect where hot water freezes faster then cold water? I always figured if it was real, it had to be some momentum-based phenomenon. So if (a) hot water in a freezer sets up strong convection currents and (b) those currents are more useful for cooling the water than the "usual" reverse convection of cold water (by bringing warmer water to the surface where its less insulated, or at least not actively burying it) and (c) the regular convection is able to continue on momentum even when the water has cooled a lot, then it might conceivably accelerate cooling enough to win the race. Maybe in just enough of a patchwork, fragile way to match the mess of inconclusive tests about the whole phenomenon.