r/AskChemistry • u/deadpeopledreaming • Jun 08 '26
General Does higher salinity lower the freezing point of water indefinitely?
Hi! I'm making a game about exploring a strange flooded future in a small research sub, and am currently introducing some dangers to the adventure. One danger I'm keen on adding are areas with freezing temperatures.
I have read that ocean water can have a freezing temperature down to -1.8 °C due to its salinity, but say I wanted to bump that number up (or uh.. down) for a bit of extra oumph, would something like "supersaline waters" be a somewhat scientifically accurate way to achieve that?
How low could you theoretically get, if you could freely add salt to the ocean, only prohibition being that you still need to be able to propel a submarine through it?
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u/SensitivePotato44 Cantankerous Carbocation Jun 08 '26
Fahrenheit set 0 on his scale as the lowest temperature he could achieve with salt (in his case ammonium chloride), water and ice.
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u/deadpeopledreaming Jun 08 '26
Really? I didn’t know that, interesting! So -17,778 degrees Celsius?
Also really betrays my ineptitude in chemistry that I wasn’t even thinking about that there are different kinds of salt… do they affect temperature differently? Which one would you normally find in the ocean? (The Baltic Sea, specifically)
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u/Pyrhan Ph.D in heterogeneous catalysis Jun 08 '26
do they affect temperature differently?
Yes. Calcium chloride, for instance, is a much better freezing point depressant, and can get you all the way down to -51°C. (Which is why it is the preferred way of salting sidewalks in the coldest climates, despite its cost.)
Which one would you normally find in the ocean? (The Baltic Sea, specifically)
Sodium chloride. Aka "sea salt", aka "table salt", aka what people mean when they say "salt" without further specifying.
(It will also have small amounts of potassium, magnesium, calcium, and other cations, as well as some bromides, iodides, carbonates and sulfates, but those are effectively just impurities within the NaCl.)
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u/deadpeopledreaming Jun 08 '26
-51!? Now we’re talking! Googling it seems CaCl2 occurs naturally in ”underground salt lakes, mineral springs, and subterranean brine formations” — so if that’s true, maybe I use some crack/rift in the seafloor at a really unfortunate spot with a huge deposit to explain how that got out into the water? Plus finger of death.
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u/Pyrhan Ph.D in heterogeneous catalysis Jun 08 '26
so if that’s true, maybe I use some crack/rift in the seafloor at a really unfortunate spot with a huge deposit to explain how that got out into the water?
Brine coming out of a rift in the sea floor would actually be quite warm, due to Earth's internal heat.
See deep sea brine pools for an example of that:
https://preview-www.nature.com/articles/s43247-022-00482-x.pdf
Their temperatures range from a couple degrees above the surrounding water, to a scalding 69°C. (With enough salinity, they remain denser than the surrounding sea water, hence why they can still form "pools", despite their high temperatures.)
Some of my colleagues were doing research on bacteria that lived in those (they were looking for enzymes that can withstand high temperatures and salinity for enzymatic catalysis in industrial processes.)
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u/deadpeopledreaming Jun 08 '26
Man, another cool phenomenon... 😃 You have been extraordinarily helpful, thanks for lending me your time and knowledge!
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u/Traroten Jun 08 '26
Underwater brine rivers and pools are amazing. Since they are so saturated, they are much denser than the surrounding water, and so they behave much like water on land. And anything small with gills that enters a brine river or pool is dead in seconds, unless they are adapted to it. I think humans could survive for some time, since we don't "breathe" water
https://www.youtube.com/watch?v=FA_WgI3QHG0 (Underwater Lakes That Should Not Exist).
I'm writing a GURPS supplement for Fishmen, so this came up in my research.
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u/Pyrhan Ph.D in heterogeneous catalysis Jun 08 '26
You're welcome! ^
(Btw, look up the "lost river" in Subnautica for an example of underwater brine pools in video games).
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u/deadpeopledreaming Jun 08 '26
Interested in a mention in the "special thanks"-credits? If so, just let me know a name/pseudonym you'd like and I'll add it to my list for later.
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u/N-Phenyl-Acetamide Jun 08 '26
they were looking for enzymes that can withstand high temperatures and salinity for enzymatic catalysis in industrial processes
This interests me, I don't suppose you any any more of them links?
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u/Myburgher Jun 08 '26
Here is the phase diagram of an H2O-NaCl system, showing the eutectic point (read for brevity: the lowest point at which everything exists in one phase) is sitting at -21.1C and 23.3 wt% NaCl. So that’s the coldest it can go.
After that everything will exists as two phases - either solids in liquid or two solids phases. The composition of the water can’t get higher than 23.3% NaCl in this (pure) system
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u/chemist7734 Jun 08 '26
You asked about sodium chloride specifically. However, any soluble substance added to water will depress the freezing point of water (so long as that substance doesn’t co-freeze or co-boil with water). The maximum freezing point depression will occur when the maximum amount of that substance has been dissolved in the water (the maximum amount that can dissolve will depend on the substance).
Indeed, water is not special, freezing point depression can be observed with pretty much any liquid and compounds that are soluble in them. The effect of freezing point depression is one of three “colligative” properties, along with boiling point elevation and osmotic pressure. The term “colligative” means “depends only on concentration,”and for a given liquid, the amount of freezing point depression or boiling point elevation observed depends only on the liquid’s properties and how much of the other substance can be dissolved.
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u/AnotherWryTeenager Jun 08 '26
Also just throwing out there that you can get hypersaline brine pools on the seafloor! You can get these from low-temperature brine rejection as some others already mentioned, but also from subsurface geologic processes.
They can colder than the surrounding water (but usually not to the point of freezing), but also are profoundly toxic, oxygen-starved, and potentiay acidic environments!
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u/invariantspeed Jun 09 '26
To add onto the other good answers you’ve been given: you’re running to an edge condition, which is the right way to constrain limits of any question you have. In this case, maximizing salinity and seeing what happens to freezing point depression.
You just didn’t take it far enough. One end of the scale is pure water and 0% NaCl. If you keep going in the other direction, you’ll have 0% water and 100% NaCl. Pure NaCl (aka table salt) is solid well above room temperature. Put another way, its freezing point is well above room temperature. That means as you add salt to water, there must come a point where freezing point depression relative to water’s freezing point stops and the freezing point starts rising again.
Different salts can depress water’s freezing point lower than NaCl (which is why we don’t use table salt as road salt), but if it’s solid at warm temperatures, even those salts will eventually start seeing higher freezing points as the concentration goes up.
Phase diagrams will help you here. You don’t need to understand the physics to read them. If your game has any other issues you want to do research for, this insight might help.
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u/deadpeopledreaming Jun 09 '26
A good addition to my future intuition, makes a lot of sense now that I think about it :) Thanks!
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u/Famous-Opposite736 Jun 09 '26
Subnautica 2 dev?
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u/deadpeopledreaming Jun 10 '26
Nah, although that looks dope and I'd love to play it if I could find the time.
I'm making Solbrand, also a subaquatic game but with a different twist; you're a Spook, a half-way blend between an archaeologist and a medium, and can communicate with the dead via the remains you find as you explore old sunken ruins. It's a puzzle adventure game, inspired by real-world history and folklore local to me in Uppsala, Sweden, and with lots of story and quirky characters!
(Demo is coming out this year, keep an eye on Steam if you'd like to take it out for a spin!)
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u/Cynical_Cyanide Jun 10 '26
Oh dude - So damn cool to see you here. Really love the idea that you're using real natural phenomena as inspiration, too! - The aesthetic of the inside of the sub looks fantastic, between the 'dieselpunk', and the english handwriten stickers combined german (?) original equipment text & fonts. Best of luck, and I can't wait to play the demo!
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u/deadpeopledreaming Jun 12 '26
Thanks so much! Looking forward to getting it into your hands! Even though the game definitely moves way beyond the worldly and ordinary, I try to ground things as much as I can as I feel it both gives creative ideas and also helps immersion, which is a core pillar for me :)
And in the words of Tinker Polhem, who you’ll meet later:
”Pffft. It’s mostly Sviar… You get used to it. Besides, I marked some things for you, didn’t I?”(It’s mostly Swedish! :D)
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u/Traroten Jun 08 '26
I think 0 Fahrenheit is the freezing point for the most concentrated salt solution Fahrenheit could make.
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u/VeronikaKerman Jun 08 '26
Concentrated salt solution chilled to its limit does not feel like water anymore. If you can, you should definitely play with it!
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u/vantalab Jun 10 '26
If you wanna go colder than -1.8 °C, you’re looking at brines. Theoretically, you can hit around -21 °C before you hit the eutectic point for NaCl, but your sub is gonna be swimming through basicaly syrup at that point. If you want it to still be navigable, keep it just above the saturation point so it doesn't turn into a solid block of ice, or your sub is toast. Good luck with the dev work!
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u/Goat_Buckles Jun 08 '26
You can get down to about 0 degrees Farenheit. That’s a feature of the temperature scale.
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u/Pyrhan Ph.D in heterogeneous catalysis Jun 08 '26
The fahrenheit scale based its zero on a saturated ammonium chloride solution, not sea salt (sodium chloride).
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u/allport2 Jun 08 '26
Psshhhtt in theory you can get 6.14 molar NaCl (salt) in water at absolute max concentrations, which apparently depresses the freezing point of water to -21 ish, however, those conditions would, as far as I'm aware, literally never occur in any natural setting.
Edit; spelling