I've been racking my brain to figure this one out, I really don't understand it. If you fill a balloon with air from your lungs, just manually blow it up, it should be carbon dioxide, no? Density of Carbon Dioxide at Sea Level: 1.977 g/L Density of Air at Sea Level: 1.2 g/L So it's heavier. I know I'm pulling these numbers out of seemingly nowhere, but I'm sure someone can verify they are accurate. So why on Earth would a balloon filled with a gas heavier than the surrounding gas in it's environment not drop at the normal speed. Aren't I right to think that bouyancy only takes effect when the gas inside the balloon is at least a bit lighter? Or is there some critical property I'm missing? Please enlighten me, thank you.