Chemistry middle-school May 21, 2026

Why Does Soda Fizz?

Gas escaping from a pressurized drink

A soda bottle and glass showing carbon dioxide bubbles leaving the liquid after the bottle is opened.

Soda fizzes because it holds carbon dioxide gas under high pressure. When you open the bottle, the pressure drops, so the gas leaves the liquid. Tiny scratches, dust, or ice give bubbles places to start.

Big Idea. NGSS MS-PS1-1 connects soda fizz to particle models of a dissolved gas leaving a liquid mixture.

A sealed soda bottle looks calm, but it is storing a lot of gas. During bottling, carbon dioxide is pushed into the drink under high pressure. That pressure helps the gas stay mixed with the water. When the cap opens, the pressure above the liquid suddenly drops. The drink can no longer hold as much carbon dioxide, so gas particles begin to escape. They form bubbles, rise through the liquid, and pop at the surface. This is fizz. The same drink can fizz more or less depending on temperature, shaking, and the surface it touches. A warm soda usually loses gas faster than a cold one. A rough candy or a scratched glass can make many bubbles appear at once. Soda fizz is a good middle-school chemistry example because it links particles, mixtures, pressure, and changes that are mostly physical.

Gas is packed in

A sealed soda bottle cutaway showing carbon dioxide particles dissolved in the liquid and gas pressure in the space above it. — A sealed bottle keeps carbon dioxide dissolved.

Soda is carbonated before it reaches the store. Carbon dioxide gas is forced into the liquid while the container is under high pressure. Under those conditions, many carbon dioxide particles can stay spread out between water and flavor particles. The drink is still a mixture because the gas has not turned into a new substance. It is dissolved in the liquid. A sealed bottle also has a small space above the soda. That space is filled with gas pressing down on the liquid. The pressure helps keep more carbon dioxide dissolved than would stay there in an open cup. This relationship is called Henry's law in chemistry. At middle-school level, the key idea is simple. Higher gas pressure above a liquid lets more gas stay mixed in the liquid.

More pressure means more carbon dioxide can stay dissolved.

Opening drops pressure

An opened soda bottle showing gas escaping from the top and bubbles forming in the liquid after pressure drops. — Opening the bottle lowers the pressure.

The first hiss happens when gas rushes out of the space at the top of the bottle. Before opening, that space has higher pressure than the air around it. When the cap loosens, the gas moves from high pressure to lower pressure outside the bottle. The liquid is now in a new situation. It is holding more carbon dioxide than it can keep at normal air pressure. Carbon dioxide particles begin leaving the liquid and joining bubbles. The bubbles grow as more gas enters them. They rise because gas is less dense than the liquid around it. At the surface, bubbles burst and release carbon dioxide into the air. The soda keeps fizzing until the amount of gas left in the drink fits the lower pressure.

Fizz starts because the drink can no longer hold as much gas.

Bubbles need starting spots

A close-up of soda near a rough surface where bubbles form at scratches and rise upward. — Rough spots help bubbles start.

Carbon dioxide does not always form bubbles evenly everywhere. A new bubble needs a tiny place to begin. These places are called nucleation sites. They can be scratches in a glass, dust specks, fibers, ice cracks, or rough spots on candy. Gas particles collect in these tiny pockets. Once a small bubble exists, more carbon dioxide can enter it. The bubble grows, breaks away, and rises. Then another bubble can form at the same spot. This is why a scratched glass may show streams of bubbles from certain points. It is also why dropping a rough object into soda can make a sudden foam. The object does not need to create carbon dioxide. It can simply give dissolved gas many places to gather and escape quickly.

Nucleation sites are tiny bubble starter spots.

Temperature changes fizz

Two soda glasses comparing cold soda with fewer bubbles and warm soda with many escaping bubbles. — Warm soda releases carbon dioxide faster.

Cold soda usually stays fizzy longer than warm soda. Temperature changes how easily carbon dioxide stays dissolved. In a colder liquid, particles move more slowly, and more carbon dioxide can remain mixed in the drink. In a warmer liquid, particles move faster, and carbon dioxide escapes more easily. This is why a warm soda can foam strongly when opened. It is also why an open soda left on a warm table goes flat faster. Shaking adds another effect. It spreads tiny gas bubbles through the liquid and increases contact between gas and soda. When the bottle opens, those bubbles grow quickly because pressure drops. Cold, still soda has fewer starting bubbles and can hold more dissolved gas, so the fizz is usually gentler.

Cold liquid holds dissolved carbon dioxide better than warm liquid.

Fizz affects taste

A glass of fizzy soda and a glass of flat soda showing more bubbles in the fizzy drink and fewer bubbles in the flat drink. — Losing carbon dioxide makes soda go flat.

Fizz is not only a sound and a set of bubbles. It also changes how soda tastes and feels. Some carbon dioxide in water forms a small amount of carbonic acid. This weak acid gives carbonated drinks a sharp or tangy taste. The bubbles also create a prickly feeling on the tongue as they form and burst. When soda goes flat, much of the carbon dioxide has left the drink. Less gas means fewer bubbles, less bite, and a different flavor. The main ingredients are still mostly there, but the mixture has changed. Measuring mass can show that gas has escaped. If an open cup of soda sits long enough, it loses carbon dioxide to the air, so its mass becomes slightly lower.

Flat soda has lost much of its dissolved carbon dioxide.

Vocabulary

Carbon dioxide: A gas made of carbon and oxygen atoms. It is the gas added to soda to make fizz.
Dissolved gas: Gas particles spread throughout a liquid mixture.
Pressure: A push spread over an area. In a sealed soda bottle, gas pressure helps keep carbon dioxide in the liquid.
Solubility: How much of a substance can dissolve in another substance under certain conditions.
Nucleation site: A tiny spot where gas particles can collect and start a bubble.
Carbonic acid: A weak acid that forms when some carbon dioxide reacts with water.

In the Classroom

Cold soda, warm soda

20 minutes | Grades 6-8

Students compare equal amounts of cold and warm carbonated water in clear cups. They observe bubble rate, foam height, and how long the fizz lasts.

Bubble starter test

25 minutes | Grades 6-8

Students place clean objects with different textures into small cups of carbonated water. They look for which surfaces create the most bubble streams and connect the results to nucleation sites.

Mass of escaping gas

30 minutes | Grades 7-8

Students measure the mass of a sealed carbonated drink, then open it and measure again after fizzing slows. The class discusses where the missing mass went and why gas is still matter.

Key Takeaways

• Soda contains carbon dioxide gas dissolved under pressure.
• Opening the container lowers pressure and lets carbon dioxide escape.
• Bubbles often begin at scratches, dust, ice, or other nucleation sites.
• Cold soda holds carbon dioxide better than warm soda.
• Soda goes flat when much of its dissolved carbon dioxide leaves.

Sign in to save