Here's an even more interesting physics brain teaser: Imagine you put a reverse osmosis membrane at the bottom of the ocean and connected it to a pipe filled with fresh water leading to the surface. The pressure at the bottom of the pipe would be less than the pressure at the bottom of the ocean, since fresh water weighs less than salt water.

In this setup, would an endless supply of fresh water flow through the membrane and bubble out the top of the pipe? I'm guessing not, but I'm having a hard time understanding why.

Wild guess: maybe only the partial pressure[1] controls whether water will flow through the membrane, and the partial pressure of freshwater in this scenario is identical on both sides?

[1]: https://en.wikipedia.org/wiki/Partial_pressure

> I'm guessing not, but I'm having a hard time understanding why.

Second law of thermodynamics.

Laws of thermodynamics aren't really useful here. They are perfect for explaining universe, and notoriously bad for predicting local events when you zoom and focus at a fine grain level and add time. Life beings are apparently breaking or "delaying" this laws all the f*ng time.

In any case nobody is trying to make a perpetual movement machine.

The goal is to create a cheaper way to extract salt from saltwater, and use the pressure gradient at the sea to put apart brine and water may be a solution waiting for an engineer (in my opinion). It is assumed that will not be free in terms of energy. It doesn't matter as long as is slightly better than the current solution. Would be much faster than waiting for the sun so it does not need to be cheaper than that. It just needs to be able to replace the last energy-expensive phase of our current solution by another process that is cheaper or faster.

A corpse of freshwater immersed in saltwater would experience a force up because its lower density. The weight of the submarine itself cancels this but if we keep adding freshwater at some point we would cross a density threshold. Probably a big volume. Maybe too much to be practical. Dunno.

Even more, ice floats so in a case of live or death if we could freeze with liquid nitrogen or so a big enough volume of cold water while avoiding the effects of the increase in volume, in theory the submarine could emerge automatically. We can't do it in the main submarine (would explode and the non frozen parts would implode immediately), but maybe in an independent storage area attached and able to absorb the extra volume?... dunno

A way to lower the temperature just when the oxygen is about to end would add also some precious extra time. A corpse is dead only when is warm and dead. In any case I'm just digressing wildly about an extreme and hypothetical emergency case. I could be totally wrong or not practical. I prefer not to test it.