A question of liquid and gases

[Deseoso]

I have a silly question for which I have no practical application for the knowledge other than satisfying my own curiosity. I go camping a lot and sometimes if I am camping and it is very rainy outside I may opt to urinate into an empty Coca-Cola bottle (plastic NOT glass) rather than leave my tent and get wet. I will then close the lid tightly and put the bottle somewhere out of the way.

 

A while later I tend to notice that the sides of the bottle have imploded a little as though some of the air from the bottle has been removed. Why is this?

 

At first I thought that the heat from the urine would cause the air in the bottle to expand pushing some of the air out of the bottle but I seal the bottle a tightly as I can as I don't particularly want the contents coming out in my tent. Would the expanding air from the urine's heat be strong enough to squeeze out of the bottles seal or is there another explanation I haven't considered?

[CaptainPanic]

Your urine is 37 degrees Celsius. At that temperature, the vapor pressure of the water is about 0.06 bar. And your urine will heat the air and possibly the rest of the bottle too... and indeed, some water will evaporate. Since the bottle is open at the top, the total pressure is still gonna be the same as everywhere else... so, since you have 0.06 bar of water vapor in the bottle, you will only have 0.94 bar of air.

 

Then you close the bottle. No air or water vapor can go in or out.

 

And then you leave the bottle, and the temperature in the bottle drops. This means that the vapor pressure of the water (or urine) is now less. Let's assume the temperature drops to 15 degrees Celsius. The vapor pressure is around 0.02 bar at that temperature. So, if you had a very sturdy container, the total pressure would be 0.94 + 0.02 = 0.96 bar. But the bottle is not strong, and instead it just 'implodes', because the pressure outside the bottle was higher than inside, and the bottle was too weak to resist it.

 

Vapor pressure is a very strong function of temperature. At 15 degrees, only 2% of the air above water will be water vapor, but at 100 degrees the water boils, and steam will replace ALL the air. So, if you repeat this experiment not with pee, but with very hot water (don't burn your fingers!), the implosion of the bottle will be more severe. (Also, make sure your plastic can withstand the hot water - some plastics melt below 100 degrees!).

 

So, no air escaped. Instead, water vapor condensed, and liquid water requires a LOT less volume than its vapor.

 

Satisfying curiosity is what we do at this forum. Welcome!

[Deseoso]

Curiosity satisfied. Thank you. Now I can stop thinking of urine bottles and my brain can start thinking about the next totally impractical and pointless question that peaks my interest.