Fizzon

Meet Fizzon, a sealed soda bottle who keeps his fizz inside by keeping the pressure up.

Gases can dissolve in liquids. How much dissolves depends on the pressure of that gas above the liquid. Squeeze more gas molecules into the space above the drink and more of them push their way into it.

Henry's law says the dissolved concentration is c = k_H p. For carbon dioxide in water at 25 °C, k_H is about 0.034 mol per litre per atmosphere. A bottle sealed with about 3 atm of carbon dioxide therefore holds roughly 0.1 mol of the gas in each litre, about 4.5 g.

Open the cap and the carbon dioxide pressure above the drink falls to almost nothing. The drink now holds far more gas than it can keep, so the extra comes out as bubbles.

The same law explains why divers can get the bends. Breathing high-pressure air dissolves extra nitrogen in the blood, and coming up too quickly lets it form bubbles.