Why do they use CO2 in fizzy drinks?

There are a number of reasons for using carbon dioxide - CO₂ - in fizzy drinks...

One of them is that carbon dioxide dissolves really quite well compared with nitrogen. Nitrogen will barely dissolve: it's very insoluble; you have to make nitrogen work quite hard to dissolve and that's why when you get the bends after you surface quickly from diving - it's the nitrogen that bubbles out of your blood and causes the bends because it just doesn't want to dissolve.

But carbon dioxide - CO₂ - does dissolve, which is critical for making a drink fizzy.

The other thing that carbon dioxide does when it goes into water - and one of the reasons it dissolves quite well - is it reacts with water to make carbonic acid - H₂CO₃.

So, CO₂ plus H₂O goes to H₂CO₃, that's carbonic acid. That carbonic acid dissociates into H⁺ (that's the acid bit, the hydrogen ions) plus HCO₃^- - bicarbonate. When you have acids in a liquid, acids taste "lemony". So you get this very nice lemon flavour added to the drink. 

So the carbon dioxide not only dissolves well so you can get your drinks really fizzy and get your gases in, it also means it tastes nicer. And you can get lots of gas dissolving so it comes out gently in the drink. It stays fizzier for longer.

Also it's free. You can get carbon dioxide from the brewing industry - yeast produces it as a waste product - so and you don't even have to purify it. You can just get it off the yeast and use it to make your drinks fizzy...