how long is air breathable for? question from the movie "lady in the water"

I saw the movie “lady in the water” last night. I really didn’t like it but it raised a couple questions…
one of them can be answered here (I will try to ruin as little as possible for those who intend on seeing the movie):
in one scene, one character swims to the bottom of the water… there are glasses which are flipped over. as he starts to run out of air, he takes a glass and breathes the air which is trapped in the upside down glass through a straw.

the context of the movie leads you to believe that the glass had been down there for some time. Obviously, it was ‘stagnant’ air. wouldn’t air lose certain properties after a certain amount of time or can the air trapped in a small contained environment always be breathable and have the same properties?
sorry if this question is confusing… but so was the whole movie. I think that if you see the movie, you will get a better understanding of the question I’m trying to ask.

I know the scene you’re talking about, and I wondered too, but oxygen is already an element and can’t break down into any more components, so I don’t see any obvious reason why it wouldn’t be breathable.

The possibilties I can think of are:
– The water was not saturated with oxygen yet, and the ability of water to absorb oxygen is greater than it’s ability to absorb other gasses, in which case the air might be lacking in oxygen. I doubt that that’s the case, however, unless for instance the air has been sitting there for weeks. Someone can probably correct me on that.
– The air might grow mold or other unsavory living stuff? Can’t say if that’s true or not, but both of these seem sort of out there, possibility wise.
– The air absorbs stuff from the water…perhaps if the scene takes place in a swimming pool it could have chlorine in it? If it’s a chlorine pool and I stored air there that would be the only thing I’d worry about (that and the air not being there in the first place!)

I don’t know the relative weights of these possibilities, though.

Atmospheric air is basically just nitrogen and oxygen with very small amounts of other stuff. The percentage of oxygen in air is about 20% throughout the atmosphere. Oxygen doesn’t just vanish; it’s a stable element. If the air had oxygen in it when it was trapped down there, it still would have had oxygen in it when the guy tried to breathe it. The only way it would have lost its pxygen is if something in the glass consumed it, like fire or some biological process.

Or it could dissolve into the water. I wasn’t able to find anything useful in Wikipedia about this. I presume that in the absence of other chemicals, oxygen dissolves into water quite readily, because algae forms in it. Algae use photosynthesis, which requires oxygen.

The pool water is probably acidic from dissolved chlorine, which prevents algae build-up (I think). Whether this retards or promotes oxygen solvation, I have no idea. I suspect that traces of chlorine gas are in the trapped air.

Nitrogen and oxygen are next to each other on the periodic table, and my stupid guess is that their solubility in water is not that much different. So, if there’s anything left in the glass at all, it’s probably air in the standard ratio of nitrogen to oxygen. So if anything is left in the glass, it’s breathable.

I have no intention of seeing this movie, but I have to ask if it’s possible for an upturned glass to remain undisturbed at the bottom of a pool for a period of weeks.

You can get a feel for the rate at which that might happen by playing with an Oxygraph dissolved oxygen monitor.
If you put a sample of deoxygenated water in the unit, and leave the cap off so it’s open to the air, you’ll see the dissolved oxygen concentration rise slowly; so that after 20 minutes or so, it’s still less than half the concentration of that in unsparged water.
If you then start stirring your sample of oxygen deficient water, it’ll suck O[sub]2[/sub] out of the air and reach equilibrium within a couple minutes.

If the pool is properly aerated, the air in the glass won’t lose oxgen to it because the system is already at equilibrium. If the pool is anoxic, the quality of the air in the glass will depend on how long the glass has been submerged, the size of the air water interface within the glass, and whether that interface has been disturbed by currents or vibration.