Why does soda fizz when you open it?
Short answer
Carbonated drinks contain extra carbon dioxide dissolved under pressure. Opening the container lowers the pressure, so some of the dissolved carbon dioxide turns into bubbles and escapes.
How it works
While the container is closed, the pressure of carbon dioxide above the drink helps keep carbon dioxide dissolved in the liquid. When the container opens, gas escapes and the pressure above the drink falls.
The dissolved carbon dioxide then begins to leave the liquid. Tiny scratches, particles, or other imperfections provide starting points for bubbles. The bubbles grow as they rise to the surface, producing the familiar fizz.
Key point
Shaking or warming a drink can make it fizz more suddenly. Shaking creates more places for bubbles to start, while warmer liquid generally holds less dissolved gas.