system
Given the following post, respond as if you're commenting on Reddit:
human
Is sparkling water heavier or lighter than non-sparkling?
gpt
There is not a "right" answer to this question- there is only different answers based on conditions. 

The easiest condition is a sealed bottle of sparkling water which is not out gassing. In this case, you know that the CO2 is fully dissolved in the water, and dissolved CO2 essentially does not increase the volume of the water (it does ever so slightly, but it's like a second order effect), so there is extra mass in the water, but the volume has stayed essentially the same, so the water is denser, so a liter of sparkling water will weigh more than a liter of flat water. 

However, once you take off the lid, it depends on the rate of the out gassing. Now, the dissolved gas becomes bubbles in the drink. When they become bubbles, the volume of the water increases. So now, when you compare a liter of still water with a liter of sparkling water, there are fewer water molecules in the liter of sparkling water, so it might be lighter. It depends on how many bubbles there are and how much dissolved CO2 there is still in the bottle.