Soda fizzes because carbon dioxide gas has been dissolved in the liquid under pressure.
While the bottle or can is sealed, the high pressure above the drink helps keep a large amount of carbon dioxide dissolved in it.
Open the container, and the situation changes immediately.
Pressure keeps more carbon dioxide dissolved
Gases can dissolve in liquids. How much stays dissolved depends on factors including pressure and temperature.
During carbonation, manufacturers put carbon dioxide into the drink under elevated pressure. The sealed container helps maintain that condition.
When you open it, the pressure above the liquid suddenly falls toward ordinary atmospheric pressure.
The drink can no longer hold as much carbon dioxide in solution, so gas begins escaping.
That escaping gas is the fizz.
Why do bubbles start on scratches and surfaces?
A bubble needs somewhere to begin.
Tiny scratches on a glass, fibers on a straw, bits of ice, sugar crystals, or other rough surfaces can provide nucleation sites where carbon dioxide molecules gather.
Once a small gas pocket forms, more carbon dioxide can enter it. The bubble grows, rises through the drink, and releases its gas at the surface.
That is why bubbles often seem to stream from particular spots on the inside of a glass.
The glass has tiny launch pads.
Why does shaking soda make such a mess?
Shaking creates many small gas pockets and mixes gas throughout the liquid.
Open the container immediately afterward, and the pressure drops while all those potential bubbles are ready to grow.
Carbon dioxide expands and escapes rapidly, pushing liquid along with it.
The soda did not become more carbonated because you shook it. You simply created a much better escape plan for the gas already there.
Why does warm soda foam more easily?
Carbon dioxide is generally less soluble in warmer liquid than in colder liquid.
That is one reason cold soda tends to hold carbonation better. Warm a carbonated drink, and the gas is more eager to leave the solution.
Temperature and pressure work together, which is why a warm, shaken can is not an experiment you need to repeat for science.
Why does soda eventually go flat?
Once the container is open, carbon dioxide keeps moving out of the drink until the system approaches a new equilibrium with the surrounding air.
Every bubble that escapes carries away some carbonation.
Resealing a bottle slows the loss because pressure can build again in the headspace, but it does not perfectly restore the original factory conditions.
Leave the drink open long enough and most of the extra dissolved gas leaves.
The result is flat soda.
Does the carbon dioxide affect anything besides bubbles?
Yes. Some dissolved carbon dioxide reacts with water to form carbonic acid, which contributes a little to the tart character of carbonated drinks.
Many sodas contain other acids too, so carbonation is only part of the flavor chemistry.
But the visible show is straightforward.
Carbon dioxide is packed into the drink under pressure.
You open the container.
The pressure drops.
The gas leaves with bubbles, noise, and occasionally your dignity if you shook the can first.
A related bit of kitchen science appears in What is pH?, where a different familiar food has its own chemistry.
And that’s the short explanation.
Related explanations: What is pH?, Why does popcorn pop?, and Why does organic milk last longer?