How Carbonation Works: Why Soda Fizzes and Goes Flat
Carbonation dissolves carbon dioxide gas into water under pressure. When the bottle is opened, the pressure falls and the gas escapes as bubbles, which is the fizz.
Carbonation is the process of dissolving carbon dioxide gas in a liquid, usually water, under pressure. While the bottle or can stays sealed, the gas stays dissolved. Opening the container lowers the pressure, so the gas leaves the liquid as bubbles, and that release is the fizz.
What happens inside a sealed bottle?
A sealed bottle has three parts that matter: the liquid, the gas in the space above it, and the cap that holds the pressure in. Carbon dioxide molecules in that space constantly strike the surface and dissolve, while dissolved molecules escape back into the gas. When the two rates balance, the liquid holds a fixed amount of gas for that pressure and temperature. The carbon dioxide is the same waste gas that cells release during cellular respiration.
Henry's law describes how much gas the liquid holds. At a fixed temperature, the amount of a gas dissolved in a liquid is proportional to the pressure of that gas above it. Doubling the pressure roughly doubles the dissolved gas. A typical soft drink holds about three to four times its own volume of CO₂.
Why does the fizz escape when you open it?
Opening the cap drops the pressure in the space above the liquid to about that of the air. The liquid can no longer hold the same amount of gas, so the extra CO₂ leaves the water as bubbles.
Bubbles do not form easily in clean, smooth water. A bubble needs a starting point, called a nucleation site, where gas molecules can gather. Microscopic scratches in glass, dust, and cellulose fibres from a paper towel all work as sites. That is why a rough spot on a glass can release a steady stream of bubbles while the smooth parts stay quiet.
Once a bubble starts, it grows as dissolved gas moves into it from the surrounding liquid. It stays attached to the nucleation site until its buoyancy overcomes its grip on the surface. Then it breaks free and rises, and as it climbs it picks up more gas and grows larger. A tall glass therefore shows a column of bubbles that gets bigger toward the top.
What controls how much fizz stays in the drink?
Temperature changes how much gas a liquid can hold. Cold water holds more CO₂ than warm water, so a warm drink loses its fizz faster once it is opened. Heat also raises the pressure inside a sealed can, which is why a can left in a hot car can spray when it is opened.
The main factors are summed up below.
| Condition | Effect on dissolved CO₂ |
|---|---|
| Lower temperature | More gas stays dissolved |
| Higher pressure | More gas is dissolved |
| Opening the container | Pressure falls and gas escapes |
| Rough surfaces or dropped solids | More bubbles start at once |
How is soda carbonated in a factory?
Bottlers chill the water first, because cold liquid holds more CO₂. The water is then mixed with CO₂ under pressure in a carbonator, and the container is sealed before the gas can escape.
The gas itself is common in nature. Yeast and bacteria release it when they ferment sugar, which is how bread rises and how beer and sparkling wine gain their bubbles. Most soft drinks use purified CO₂ from industrial sources instead.
Does carbon dioxide change the taste?
Yes. Some of the dissolved CO₂ reacts with water to form carbonic acid, H₂CO₃, which makes the liquid mildly acidic. Soda water is typically around pH 3 to 5, depending on how much gas it holds, and that acidity is part of why carbonated drinks taste tart. The bubbles add a prickling feel on the tongue, so the sharp edge is both chemical and physical.
Flat soda is not chemically spoiled. The gas has simply left, and the sugar, acid and flavour remain. Once the CO₂ is gone, the drink tastes sweeter and less sharp, which is the main difference people notice when a bottle has been left open overnight. Less of the acid forms as the gas leaves, so the acid-base balance of the drink shifts too.
Tags
carbon dioxide carbonation chemistry food food science