I’m curious: once I open a bottle of club soda, and seal it again after pouring some out to drink, the next time I open it, the stuff won’t be as fizzy as the first time.
Can I keep it fizzier by keeping it refrigerated, or would it stay fizzier at room temperature? Or does it not matter at all?
Yes, it will stay fizzier longer if refrigerated but not indefintely.
The “fizziness” is caused by the CO[sub]2[/sub] gas escaping from the solution. Every time you open the bottle (or can), more gas makes its way to freedom (“Fly my brothers, be free!”)and there is less fizzy left in the container for your amusement.
The (simplified) equation PV=nRT is relevant here. (Pressure)x(Volume)=(amount of substance in moles)x(a constant)x(Temperature).
For your club soda, the terms n and R can be safely ignored. So, we are looking at the relationship between the Pressure of the gas, the Volume of the gas and the Temperature of the gas, or PV~T
Opening the container decreases the pressure. If the Pressure has gone down, then the Volume must go up. The gas expands into the room, depriving you of that neat little koosh sound the next time you pop the lid.
A rise in temperature will impart energy to individual gas molecules, allowing them to escape one by one instead of in droves. It may take awhile but even an unopened can of soda at room temperature will eventually go flat. (I found this out long ago with a forgotton can of soda that sat in our basement for, literally, ten years. Flat as a sheet of paper and never been opened.)
There are devices which should still be in production which allow you to pressurize bottles with air. Sort of a pump cap. This will keep the pressure inside the bottle high which is probably the biggest culprit in escaping CO[sub]2[/sub].