Open a bottle of fizzy drink and you hear that hiss, then watch the bubbles climb. Leave it out for a few hours and the fizz is gone, leaving a flat, sad drink. So where did the bubbles go? The drink is packed with a gas called carbon dioxide, squeezed into the liquid under pressure while the cap is on. That pressure is the jailer keeping the gas hidden in the drink. The moment you open the cap, the pressure escapes with that hiss, and the gas starts sneaking out of the liquid as bubbles, a little at a time. Once enough has left, there is no more gas to fizz, and the drink is flat. Warm the drink or drop in something rough, like a sweet, and the gas rushes out much faster. In the simulator, pop the cap and watch the bubbles begin.
Most people think a drink goes flat because the bubbles simply rise and pop. In fact the gas is dissolved invisibly in the liquid, held by pressure, and only forms bubbles and escapes once the cap is off.
What's actually happening
Everyone knows the disappointment of a flat fizzy drink. The first glass from a fresh bottle is lively and sharp; the last glass, days later, is dull and lifeless. Nothing has been added or taken away that you can see, yet the personality of the drink has drained out of it. What has actually left is a gas, carbon dioxide, and the story of how it stays in and how it gets out is a neat piece of everyday chemistry.
Carbonated drinks are made by forcing carbon dioxide into the liquid under pressure. The rule that governs how much dissolves is Henry's law, which says that the amount of a gas a liquid will hold is proportional to the pressure of that gas pushing down on it. Inside a sealed bottle, the space above the liquid is crowded with CO₂ at several times atmospheric pressure, and that pressure props a large amount of gas in solution. The cap is doing one job and one job only: keeping that pressure up. As long as it stays on, the drink and the gas above it sit in a comfortable balance, and the fizz waits in storage.
Open the cap and you break the balance instantly. The pressurised gas escapes with a hiss, and now the liquid is holding far more carbon dioxide than the thin air above it can support, a state called supersaturation. Chemistry abhors that imbalance, so the gas begins leaving the liquid, gathering into bubbles that float up and burst at the surface. This is slow at first because forming a bubble from scratch is surprisingly hard; the gas needs a rough spot, a scratch in the glass or a speck of dust, to get started, which is what a nucleation site provides. Warm the drink and the liquid can hold even less gas, so it escapes faster. Drop in a Mentos sweet, whose pitted surface is covered in millions of tiny nucleation sites, and the gas erupts all at once in a famous foaming fountain. Either way, the drink keeps shedding CO₂ until what is left matches the air, and at that point there is nothing more to fizz. The drink has gone flat.
Fizzy drink goes flat because pressure keeps CO₂ dissolved, and opening the bottle lets that gas escape until none is left to fizz.
- 1Pour the same fizzy drink into two glasses, one straight from the fridge and one left to warm to room temperature.
- 2Watch which one fizzes faster and goes flat first, then stir a pinch of sugar or salt into one to add nucleation sites.
- 3Notice that the warm glass and the gritted glass both lose their fizz quicker, confirming that heat lowers solubility and rough surfaces speed bubble formation.
Common questions
Once the cap is off, the drink holds far more carbon dioxide than the air above it can support. The gas keeps leaving as bubbles to reach balance with the air, so it goes flat whether or not you drink any.
It helps. Recapping lets the escaping CO₂ build pressure in the headspace again, which slows further loss. It will not restore lost fizz, but a tightly sealed, cold bottle stays lively far longer than an open one.
A Mentos is covered in microscopic pits, each a nucleation site where bubbles form easily. Dropping one in releases the dissolved CO₂ from millions of points at once, producing a sudden, dramatic foaming fountain.