Does Urine Absorb Atoms from the Air and Create a Vacuum in a Closed Container?

[paldin]

I have been urinating into a plastic bottle lately (I will not elaborate why), and have been finding the bottle to hold a vacuum the following morning. What I find strange is that even without temperature fluctuations, my bottle produces a vacuum after several hours of being left alone. It is in an otherwise dark and stable place.

Does urine absorb atoms from the air or otherwise convert to a composition that is a lesser volume?

 

[flatmaster]

You said this happens "even without temperature fluctuations". Are you just referring to the ambient temperature? I would imagine if you cap the bottle immediately after filling it, the liquid and air trapped in the bottle would be warmer than the ambient temperature. After cooling, it would contract. Don't think there's any chemistry happening here. I would try the same thing with the same approximate volume of warm water and see what you get.

 

[paldin]

I use a 1 gallon container and have experimented with the volume before emptying it. When I introduce urine to the container in the hottest part of the day and find a vacuum the next morning when it is significantly colder, I am not surprised. It is when I introduce urine to a moderately full container during the coldest part of the day and still find a vacuum in the warmest part of the day, that has me astonished.

Unless my ≈98° liquid cools so much as to condense the contents of a mostly full 1 gallon container in an ambient temperature ≈90°, I suspect something chemical is occurring. I suppose I should calculate the volume and compare with STP and gas laws, but if urine is known to absorb atmosphere, then I could save myself that effort.

For the sake of argument, I'll warm some water (close to my body temp) and compare the difference. It may turn out to seem like a stronger vacuum than is actually present.