Which gas gives fizzy drinks their bubbles?

Carbon dioxide, dissolved under pressure - and the fizz is a demonstration of Henry's law, which says the amount of gas a liquid can hold is proportional to the pressure above it. Open the bottle, drop the pressure, and the liquid can no longer hold what it has. The gas comes out of solution and the drink goes flat.

The bubbles need somewhere to start, which is why a scratch, a speck of dust or the rough surface of a glass produces streams from specific points rather than uniformly. It is also why dropping mints into a diet cola produces a fountain: the sweets are covered in microscopic pits, each one a nucleation site, and the surfactants in the drink make the bubbles easier to form. It is physics rather than chemistry - nothing reacts.

Joseph Priestley worked out how to carbonate water in 1767 by suspending a bowl above a brewery vat, and published the method without patenting it. Johann Jacob Schweppe commercialised it. Priestley also isolated oxygen, though he interpreted it within the old phlogiston theory and never accepted Lavoisier's account - and he had his house and laboratory burned by a mob for his political and religious views, and emigrated to America.