Eli5: Why does beer do that thing when hit with another beer on its cap

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Eli5: Why does beer do that thing when hit with another beer on its cap

In: Chemistry

I assume you’re suddenly forcing a bunch of air to mix with the beer, turning it into foam. The foam takes up more space than just beer, so it expands by frothing up out of the bottle.

Beer is carbonated, that means it has CO2 gas dissolved in the liquid. Generally there is more gas dissolved then there wants to be based on the pressure of the air on top of the beer. When you tap someone’s bottle you send a pressure wave thru the beer in their bottle. This wave bounces around the bottle reflecting off the glass and in some points the pressure will be lower than normal so the CO2 escapes the liquid and makes a bubble.

If you hit on top of a bottle you create a shockwave traveling downwards. If it hits the bottom it gets reflected and interferes with the shockwave coming from the top (you can see the same effect when you create a wave in your bathtub – when it hits the wall the waves start to look different). This creates some high and some low preassure areas (like the wavepeaks in your bathtub (more water above the floor -> more preassure) ).
Why is this important you ask? What? You didn’t ask? Too bad, I will tell you anyway:
Gas dissolves in liquids, because the preassure above the liquid keeps the gas inside. When the preassure is lower, the gas leaves the liquid. In some drinks, like beer, there is so much gas inside, that it wants to leave, but the gas is surrounded by so much liquid, that it can’t escape.
In these low preassure areas the gas can actually escape the liquid and form tiny gas-bubbles. on the surface of these bubbles more gas can escape and form bigger bubbles. These big bubbles are what makes you go “dude wtf” on partys, when someone hits your beer.

Okay so.

Beer and coke are both liquids with some CO2 gas mixed within. When you put abrupt but temporary pressure at the top of these liquids, it gets distributed inside. While doing so, at some places in the liquid, there are too many particles and hence a high-pressure zone, while at other places there’s a low pressure zone ( this is how pressure travels inside a fluid).

So now at certain points within the fluid, we got low pressure zones, where the pressure might be so low that CO2 trapped in its surroundings will usher into it, making a bubble which grows bigger and bigger, and becomes lighter and lighter, eventually moving to the top of the container (like how things lighter than water float in it).

P.S.: a fluid doesn’t have to be carbonated for this to happen. It can be 100% pure and still this can happen. In that case, instead of the CO2, the liquid itself converts into its gaseous state forming the bubbles we talked about ( this happens when pressure at a point in a liquid drops below it’s ‘vapor pressure’). This process is apparently called Cavitation.