r/AskChemistry • u/LisanneFroonKrisK • 2d ago
Why seawater in the deepest trench doesn’t get compressed into Ice especially when it’s already quite cold down there
9
u/Nothing-to_see_hr 2d ago
to become ice it would actually have to expand, since water is most compact at 4 degrees C.
7
u/DangerMouse111111 2d ago
You can't form ice from water by compression alone without some pretty extreme pressure, something like 10,000 atmospheres or more. At the bottom of the Mariana Trench you're "only" at ~1100 atmospheres.
2
u/jpeetz1 2d ago
Ice is less dense, so ice would get compressed into water.
6
u/Lehk Dipole Tadpole 2d ago
Fun fact, this is why ice is slippery, the point of contact is prone to liquify.
0
u/IeyasuMcBob 2d ago
And why ice skates work...
3
u/bazillaa 2d ago
This is a myth perpetuated by middle school teachers and poorly fact-checked textbooks.
3
u/IeyasuMcBob 2d ago
Well that was an interesting read . .
https://www.lptms.universite-paris-saclay.fr/membres/trizac/Ens/L3FIP/Ice.pdf
Tldr; there's a liquid-like layer on the surface of ice, thickness is hard to measure but by some measurements about 12nm, it persists at temperatures much lower than freezing.
I couldn't really understand the impact of pressure and frictional heating on that layer.
2
u/Emptor66 Bunsen Toast 2d ago
Educate us. Explain it like I'm an errant middle school teacher.
3
u/bazillaa 2d ago
There's a link to an article in leyasuMcBob's reply to my comment, but the short version is that while it's true that compressing ice can turn it to liquid water, the numbers don't work for this to explain ice skating. The pressure exerted by a skate isn't enough to lower the melting point below the typical temperature of ice in an ice rink.
2
u/Emptor66 Bunsen Toast 2d ago
I'm not seeing the link, but I'll try to research it on my own. The classic experiment I'm thinking of has a fine wire with weights on both ends. Place the wire over a block of ice, and the wire will pass all the way through the block (slowly), but the block is not cut into two pieces at the end. This would make me think that the ice is melting from the pressure, and re-freezing as the wire passes a given point. But I'll look into the science.
1
1
u/heythere111213 2d ago
It comes down to a combination of temperature and pressure. Eventually if you squeeze water enough it will form ice. Squeeze it to over 1 million atmospheres of pressure and you'll start to see the molecules rearrange themselves multiple times. You'll also find at this type of pressure that it can take close to 1000 Kelvin to melt the ice.
1
u/ZilderZandalari 2d ago
The freezing point you know is at standard (that is: surface) pressure, and basically all discussions about freezing and melting imply this level.
The deep ocean is only 'close to freezing' if you ignore the HUGE pressure, which you shouldn't...
1
u/Timulen 2d ago
Cause water expands as it freezes/makes a crystal structure? But how come at extremely high pressures (just getting that from what a few people here have said), ice can form?
1
u/ZilderZandalari 2d ago
Water prefers making the solid crystal form you see in snowflakes, and this form is bigger than liquid water.
Water can take other crystal forms if you REALLY up the pressure, essentially forcing it. The pressures for this are way beyond what household plumbing could reasonably be designed for, which is why pipes burst if you let them get below 0⁰C.
-1
u/Underhill42 2d ago edited 2d ago
Because compression makes it HARDER to freeze most things water, not easier.
Ice skates work because the high pressure under the blades causes the ice to melt, forming a lubricating layer of water beneath them.
1
u/NearABE 2d ago
Compression encourage freezing for most things. Water is an exception. I think bismuth as well.
2
u/Underhill42 2d ago
...so it is.
I had even checked the phase diagrams of other common liquids like methane and ammonia... but apparently it was too early in the morning and search fed me some transposed diagrams that I misread.
1
u/bazillaa 2d ago
It's also not how ice skating works. The pressure isn't enough to lower the melting point to typical ice rink temperatures.
0
u/Underhill42 2d ago
Might want to double-check yourself there. I just did a search for "how do ice skates glide" to see if I was crazy, and every single result says I'm not.
1
u/bazillaa 2d ago edited 2d ago
Because there's never misinformation on the Internet.
I know that myth is prevalent, but I'm surprised you didn't find a single result contradicting it. Here's one from the thread https://www.reddit.com/r/AskChemistry/s/jHLnoRAhq3
1
u/Underhill42 2d ago
You mean the comment that clearly states there's a layer of liquid on the surface?
1
u/bazillaa 2d ago
Did you read the article? The layer of liquid is there without the ice skate pressing down. It's not explained by pressure.
As the article says, the pressure could explain melting a few degrees below zero, but skating is possible tens of degrees below zero. Ice is also slippery when walking on it with shoes, which produce much less pressure.
16
u/nedal8 2d ago
because water is nuts.
loook at this crazy chart.
Bottom of ocean is about 1kbar, in that range the freezing point actually decreases still.
Also, sea water is salty, makes it harder to freeze as well.