Silly science question.

[ravells]

I've just noticed something and I wonder if any of you science boffins could tell me why this occurs. This is what I did:

1. Filled to about 90% an empty 50cl bottle of Evian water with tap water and screwed the lid back on .

2. Put it in the freezer so it became a solid block of ice.

3. Let the ice melt without taking the top off. I put the bottle in front of a deskfan so it acts as a cheap air conditioner.

4. The bottle has been on the desk for a couple of weeks.

I found that the bottle had compressed significantly, (as if someone had squeezed the bottle slightly and put the top back on). It was as if some of the air inside it had been lost.

How is this possible? If the seal on the lid was airtight then how did air escape to cause the decompression? If the seal was not airtight then why didn't the outside air pressure normalise the pressure inside the bottle?

I know there must be lots of more important things to worry about in life - I'm very curious how this happened.

I have an A level in physics (not sure what that translates to in US terms - probably something like 1st year degree) but that was 30 years ago. I'm familiar with the principles of Boyle's law but that doesn't explain to me what happened here.

Hmm perhaps I ought to repeat the experiment...

 

[Dragoner]

The expansion of the ice forced air out, then as the ice contracted back to liquid, the vacuum sealed the lid and the bottle compressed. (imo)

 

[BytePro]

Yep - what Dragoner said - though to clarify, by vacuum I believe he meant reduced pressure caused a vacuum effect (pressure differential that resealed the lid).

And even after the seal (reseal) is broken, and the pressure equalized with the ambient, the bottle will probably require additional energy (i.e. be deformed till you press it back) to return to its prior shape - as some energy has been lost in the freeze/thaw, unsealing, deformation and pressure changes.

 

[ravells]

Thanks guys that makes a lot of sense, particularly as plastic deforms quite easily. So I guess the screwtop almost acts like a valve which allows air out but not back in again.

Cheers!

 

[Enoki]

More likely the air, if the bottle was air tight, dissolved into the water to some degree. A change in that amount would account for the change in air pressure in the bottle.

 

[Carlobrand]

Yuppers, under the increased pressure caused by icing water compressing the air, your airtight seal ceased to be completely airtight, but it was plenty airtight when the pressure inside the bottle later dropped.

Here's an interesting if possibly messy experiment: repeat what you did, but put the bottle on its side in the freezer so that only water is against the lid. Will the increased pressure squeeze water out the lid - which you would see as a little frozen icicle dangling from the base of the lid? Or will the bottle balloon up or even fail?

 

[Timerover51]

Yuppers, under the increased pressure caused by icing water compressing the air, your airtight seal ceased to be completely airtight, but it was plenty airtight when the pressure inside the bottle later dropped.

Here's an interesting if possibly messy experiment: repeat what you did, but put the bottle on its side in the freezer so that only water is against the lid. Will the increased pressure squeeze water out the lid - which you would see as a little frozen icicle dangling from the base of the lid? Or will the bottle balloon up or even fail?

If you do that the bottle balloons out but does not fail. We did that with our kids as a home physics experiment. Another one was putting an half-empty bottle in the sun to get nice and warm, then putting the cap on and then putting it into the refrigerator. Deforms really nicely.

 

[Spinward Scout]

Plastic also contracts when cold making the lid not as airtight. I'm assuming the bottle was plastic.

 

[dean]

Plastic also contracts when cold making the lid not as airtight. I'm assuming the bottle was plastic.

Indeed; in addition the bottle cap, thicker than the bottle itself, likely did not contract as much as the bottle, allowing the slight overpressure to escape, but resealed at room temperatures.



Edit: Just noticed this is 1,000th post here. Funny!

 

[Enoki]

Oh, another thing: The bottle cap is typically styrene. The bottle is low density polyethylene. Two different plastics with different properties.