Why Did My Milk Bottle Shrink in the Heat?

[Archosaur]

I observed something interesting yesterday. At first, I couldn't explain it, but I figured it out after a few minutes, and I figured it might be interesting enough a problem to pose here.

Yesterday, on my way to work (at about 7 in the morning) I picked up a pint of milk from UDF and drank it in the car (cause I got to have strong bones). When I finished it, I put the cap back on and tossed it in the passenger seat. The next time I saw it was when I was cleaning my car out at about 10 at night, after a long, very hot day. Of course, it was night at this point, so it was a little cooler now than it had been in the middle of the day, but it was still significantly warmer than it had been that morning, so imagine my surprise when I saw that the milk bottle had contracted!

How is this possible?

 

[LostConjugate]

It contracted in warmer weather?

What was the elevation difference between when you sealed it and when you saw it contracted?

When I go to Tahoe in the winter stuff blows up (bags of chips and such) because it expands even though the air is significantly cooler. It is the elevation.

 

[Archosaur]

A good guess, but go ahead and assume equal elevations. There isn't a trick to this. I've told you everything you need to know.

 

[tyroman]

OK... I see two possible mechanisms.

First, biology... if there were microbes introduced into the residue of milk before you sealed the bottle, they might have used some (or all) of the oxygen in the bottled air to metabolise some of the milk - thereby reducing the air pressure inside the bottle by an amount equal to the original partial pressure of oxygen in the air. This assumes that whatever microbe used the oxygen did not produce CO2 at a partial pressure equal to or greater than the O2 partial pressure.

Second, mechanics... essentially an imperfect seal at the cap which acted as a "check valve" and allowed hot - high pressure air to escape from the bottle while restricting air flow in the opposite direction as the hot air in the bottle later cooled. This effect might be enhanced if the materials at the bottle/cap interface (plastic?) are more flexible at higher temps.

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[Archosaur]

tyoman, those two ideas are the same ideas I initially thought of. I ruled out the biological idea because the only biological processes I know of would have increased the pressure in the bottle (though I am no biologist and I would be thrilled if someone knew of such a process).

I settled on the idea that the bottle lost some air due to the extreme heat in the middle of the day and a sort of "check-valve" mechanic, but not because of the heat affecting the flexibility of the plastic. I think it just comes from the fact that relatively high pressure inside the bottle makes the seal worse, and relatively low pressure inside the bottle makes the seal better. See what I mean?

 

[LostConjugate]

That sounds about right. Cool.

 

[tyroman]

I agree... the internal pressure would tend to lift the seal and potentially resulted in a much greater delta P, while the max delta P in the other direction is 1 atm.

On the biology issue... what about an anaerobic microbe consuming Nitrogen from the air with a solid nitrate as a byproduct? like you, I am no Biologist!

.

 

[K^2]

Except for things that photosynthesize, I am not aware of any living organism that would reduce pressure in its own environment. It doesn't make sense from perspective of entropy.

But I'm no biologist either.

 

[tyroman]

How about a non-biological chemical reaction... like rust?

Some element - simply oxidizing - could bind up all the O2 into a solid oxidate which would occupy less volume than the gaseous O2 in the air... resulting in collapse of the plastic bottle... and that will be my last speculation on the event.

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[LostConjugate]

Except for things that photosynthesize, I am not aware of any living organism that would reduce pressure in its own environment. It doesn't make sense from perspective of entropy.

But I'm no biologist either.

Entropy can escape the bottle, air can't.

For example if you take a gas inside a bottle and convert it to a lower energy solid state, you would release heat, the heat would equalize with the walls of the bottle and then radiate away. There goes your entropy and now your bottle is crushed by the outside gas pressure.

 

[Archosaur]

What K^2 is saying with regards to entropy is that there are no chemical processes from which an organism can derive working energy that would not increase entropy, and that an increase in entropy tends to manifest as an increase in heat, and thus pressure, etc.

 

[LostConjugate]

What K^2 is saying with regards to entropy is that there are no chemical processes from which an organism can derive working energy that would not increase entropy, and that an increase in entropy tends to manifest as an increase in heat, and thus pressure, etc.

But the pressure can leave the bottle.

In the case of putting hot air in a bottle and taking the bottle into a cooler environment the bottle would shrink. However if you put cool air in the bottle and heat it [the air] up the bottle would not shrink when it is cooled again.

 

[Galap]

Maybe the cap was on such that it would let air out but not in (a higher exterior pressure would push the cap on tighter), so when it heated up from the cool morning, air got out, but when it cooled after that, the air could not get in, so instead it contracted.

 

[russ_watters]

That is what others were suggesting and I agree that that is the likely explanation.

 

[m.e.t.a.]

Archosaur, you say that it was very hot all day. Hot enough to soften the plastic of the bottle and cause it to sag inwards on itself? This would give the impression that the bottle had contracted. (This is supposing that the cap screwed on loosely enough that air could escape during the sagging.) Just a guess.

Edit: I still find the bottle-cap-behaving-as-a-valve idea to be the best.

 

[Archosaur]

m.e.t.a. I thought of that too. That's why I opened the bottle to see if it would hiss, which it did.