Which gas gives fizzy drinks their bubbles?

Carbon dioxide is forced into the liquid under pressure, and how much dissolves depends on that pressure and on temperature, which is Henry's law in everyday form. Sealed, the space above the drink holds pressurised gas that keeps the dissolved carbon dioxide in solution. Open the bottle and that pressure vanishes, the liquid is suddenly holding more gas than it can, and the excess comes out as bubbles. Bubbles need a starting point, a scratch, a speck of dust, a rough patch on the glass, which is why they stream from particular spots and why a rough surface fizzes more violently than a smooth one. Cold liquids hold gas better, so a warm drink goes flat faster and foams more when opened. Some of the dissolved gas reacts with water to form carbonic acid, which is why sparkling water tastes faintly sharp rather than neutral.

The same gas is produced by yeast during fermentation, which is how naturally sparkling beer and champagne get their bubbles.