Whenever I finish a drink of water bottle, I screw the cap back on the plastic bottle and put it in the fridge. The next time I unscrew the cap, even just a little bit, the bottle expands as if the air outside the bottle comes rushing in.
I wonder why this is the case? When I finish a drink and put it back in the fridge, the bottle doesn’t appear to contract or anything. But the next time I open it, it expands as if it slowly contracted a little bit while in the fridge, then abruptly expands to its original size when I leave it there for a while and open it again.
Hope that makes sense! It’s just a regular plastic bottle like this picture.
Best Answer
Start with a room temperature bottle. It is almost entirely filled with water. You drink some and then screw the top back on. There is now some air in the head space of the bottle. This air is at room temperature and pressure.
Now place the bottle in the fridge. If sealed well, all the air that is in there is all there is. As the temperature drops, the classic ideal gas law of $PV = nRT$ holds (pretty well for air also). $n$ is constant, $R$ is a constant, so $PV$ must change proportional to $T$.
Looking at the bottle, it is designed to keep water in, so if just $P$ changed you can be sure that $V$ will reduce to some extent as well, with just how much of $P$ or $V$ changing is a tradeoff based on the mechanical properties of the bottle.
So, you go to open the now-cold bottle back in room temperature. The air outside is higher pressure, and can go back into the bottle. The volume was reduced with the pressure, so it can now expand back out towards 'normal' size.
No, recap the bottle, but leave at room temperature for a while. The water takes some time to come up to room temperature from being cold, so it re-cools the air in the head space (now larger since you drank more from it), and will expand more dramatically since there was more air-to-water volume inside the bottle.
If you have a solid bottle (Hydroflask, glass bottle), where $V$ doesn't change that much since it is a sturdy container, you will find a lot more air moving since the pressure $P$ is now solely in charge of responding to temperature changes. The same thing holds for the closely-related question just asked about changing altitude - you plastic bottle will crumple as $PV$ changes, while a solid bottle will have a larger change in $P$. Unscrewing the top of a well-sealed Hydroflask that had hot coffee in the morning is often quite hard, since the pressure drop as it cooled while empty of liquid pulls the lid down deeper onto the threads.