How come when you hit a beer bottle on top of another it causes it to overflow?



Seen this happen many times, never wondered about how it works till this moment.

In: Physics

Beer is a carbonated beverage. That means there is CO2 dissolved into the liquid well beyond its normal capacity to hold it. The CO2 will slowly escape until it reaches an equilibrium, but under normal conditions, this doesn’t all happen at once.

When you smack the bottle down, the bottle moves down very quickly. However, the liquid inside doesn’t. This creates a vacuum under the liquid. This vacuum allows the CO2 in the bottle to dissolve out very quickly, helping to fill that vacuum. Then, as the liquid actually drops down, the CO2 will form a lot of bubbles, since it will essentially form clumps of gas in the liquid. This creates the rapid foaming.
This video explores the bar trick of breaking the bottle by doing the same thing but with water in it, and they take a tangent on the beer-filled bottle.

You’re causing the carbon dioxide to nucleate, which sort means that the dissolved gas bubbles grow bigger. As the bubbles become bigger they are much less dense than the liquid around them so the rise to the surface pretty fast. As they do they bring up some of the liquid with it as they push up and this spills out of the bottle.