# Air heavier than water? Will it sink?

The seawater in deep oceans have a density of 1050 kg/m³ or so.
The density of air is about 1.3 kg/m³, about 800 times less.
If air is compressed to a density higher than the water in deep oceans and released there, will it sink to the bottom then?
I know that Boyle's law is not accurate at high pressures, but air compressed to 1/800 of its original volume will give the necessary density anyway.
Possibly the air is not a gas at that pressure and possibly it would go in solution in the water too, but if it didn't (use a balloon?), would it then be possible to fill the bottom of a deep ocean trench somewhere with air?

jbriggs444
Homework Helper
The required pressure is rather more than can be maintained by an ordinary "balloon". If you had balloons constructed of some material of negligible mass and thickness and of unlimited strength, if you filled them with air compressed to the requisite density and if you didn't run out of air then sure, you could fill a trench with balloons and they'd sit there on the bottom.

Whether air is a solid or gas under the required conditions is not particularly relevant.

An ordinary balloon should do the job if it is filled at the relevant deep, would'nt it?

Borek
Mentor
You would need to check exact density of the compressed air - my bet is that it won't get higher than the density of the liquid air, which is lower than the density of water.

olivermsun
You would want to do a check with Boyle's law. The bottom of a deep ocean trench would be beyond 8000 m depth, so the pressure would be greater than 800 atmospheres. Assuming for the moment that Boyle's law were still working, the pressure would actually compress your 1/800-volume balloon even further and cause it to sink!

olivermsun
You would need to check exact density of the compressed air - my bet is that it won't get higher than the density of the liquid air, which is lower than the density of water.

Liquid oxygen is both denser and more compressible than water, at least at around 100–150 atm. I have no idea what happens at 800 atm, though…

I have some experience with oxygen at higher pressures and it is really unpleasant to handle. It reacts with almost anything.

Borek
Mentor
Liquid oxygen is both denser and more compressible than water

Yes, but nitrogen is much lighter, at around 0.80 g/mL. Assuming they mix more or less ideally, I would expect density around 0.9 g/mL. That gives a 10% margin which makes me feel I am on the safe side.

Doesn't mean I am OK, as I stated earlier, the only sure way of knowing is to check tables.

olivermsun
Yes, but nitrogen is much lighter, at around 0.80 g/mL. Assuming they mix more or less ideally, I would expect density around 0.9 g/mL. That gives a 10% margin which makes me feel I am on the safe side.

Doesn't mean I am OK, as I stated earlier, the only sure way of knowing is to check tables.
From what little I know about the production of liquid O2, the liquid O2 and N2 tend to separate due to the density difference. So even better — you'll get the N2 rising and the O2 sinking.

Chestermiller
Mentor
The critical temperature of air is about -140 C. This is certainly lower than the temperature at the ocean depths. So air could not be liquified at these depths, no matter what the pressure.

Chet

But will it sink?

Chestermiller
Mentor
You need to consider two things. For it to be neutrally buoyant, the densities have to match. But, even if this is satisfied, you also have to consider the pressures. If the compressed air pressure is higher than the water pressure, the air will expand, and then its density will be less so it will rise. So, at the ocean depth, what is the water temperature and the water pressure? For the air is at this temperature and pressure, what is its density, and how does this compare with the density of the water? You can get the air density by considering the non-ideal gas behavior of the air. Just look up the compressibility factor z at the reduced temperature and reduced pressure (temperature and pressure scaled to the critical properties of air). Then ρ=zpM/(RT).

chet

russ_watters
Mentor
But will it sink?
What you ask is impossible, so no.

Borek
Mentor
The critical temperature of air is about -140 C. This is certainly lower than the temperature at the ocean depths. So air could not be liquified at these depths, no matter what the pressure.

I never said it will be liquified. I said "I don't expect the compressed air to have density higher than the liquid air".

Chestermiller
Mentor
I never said it will be liquified. I said "I don't expect the compressed air to have density higher than the liquid air".
Oops, sorry. My misinterpretation. Please accept my apology.

In any event, the water at those depths won't be saturated with air, so all the air will easily be able to dissolve at the high pressures.

Chet