Why water in the arctic that’s below freezing, not frozen?

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I was watching a documentary about a team that was planning to go down to the lowest point on the ocean floor somewhere in the arctic.

While the gentleman in the sub was going down, the narrator stated “at this depth, the water outside the submarine is below freezing”.

Soo… why is it not frozen? Does it have to do with the salt content in the water?

ELI5, thanks!

In: Earth Science
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It wet. Cold wet is ice only when wet too cold to be hard. No need ice when big water. Little water make small ice because small ice not enough wet for hold water. When big water it not easy make hard. Much big water stays water. Yes.

Salt lower the freezing point of water. In the same way that salt is used as an ice melter for sidewalks in winter.

Salt water freezes at a lower temperature then fresh water. This is for example why people use salt in the winter to thaw ice. But this also allows salt water to be cooled to a bit under freezing without actually forming ice.

Because its not below freezing, you’re right that if it was below freezing then it should turn into ice

Water in the deep ocean is very close to 4C always. Water is a bit weird in that it expands when it tries to freeze so water need the freezing point *floats* on water slightly above the freezing point rather than *sinking* like you would expect

Water is densest at 4C so if the water gets any cooler than 4C it’ll rise and mix with the warmer water. If its any warmer than 4C it’ll also rise. The end result is that water at peak density sinks and displaces any water that isn’t at peak density so any deep portion of the ocean is at just about the same temperature regardless of the temperature of the water at the surface

In the ocean depths, the deeper the water is the saltier it is. The more salt there is in the water, the lower the temperature it takes to freeze and when it freezes that salt moves to other parts of the water. Another thing that water needs to freeze is to be fairly still. The water at the sea floor is constantly in motion, so it doesn’t freeze.

Between the salt levels and the motion, the water in the arctic tends not to freeze below a certain depth.

Solutes such as salt cause the freezing point to be more extreme. There is actually an equation based on concentration and how many ions or pieces the solute splits into to find by how much the temperature changes for a given liquid.