Submitted by trumps_baggy_gloves t3_10ntd6y in explainlikeimfive
Surely the bottle top can't creat a tight enough seal to stop the gas particles from exiting. Especially considering how easy it it to remove the top.
Comments
the_original_Retro t1_j6arqyu wrote
Home brewer here. It's chemistry.
When they put the cap on the beer bottle, they'd already forced a lot of carbon dioxide (the gas that you breathe out) into the liquid in that bottle. They did it by placing the bottle's contents under pressure. And, under pressure, chemistry turned that carbon dioxide gas into a weak and unstable chemical called "carbonic acid" that easily breaks down under stress.
If you were to open the top, and hit the bottle a sharp knock with another bottle, WHAM. That's stress. And the carbonic acid starts to turn back into carbon dioxide REALLY FAST, and you get a foaming up mess.
But it can only do that because it's no longer under pressure. The carbon dioxide needs to go away somewhere so there's room for more to form.
That's why if you leave the cap on, and the carbon dioxide might want to come out, but it really can't... because there's no space where it can go.
As long as the bottle remains sealed, any carbon dioxide that wants to form just increases the pressure in the bottle's little air-bubble headspace, and that increase in pressure just re-dissolves the carbon dioxide back into the water part of the beer.
ArctycDev t1_j6as8kt wrote
Someone did that bottle knock thing to me once at a bar, where they thunk the bottom of their bottle onto the top of yours. I have been waiting over a decade for that to happen again so I can put my thumb over the top and see if I can make it spray like a hose at whoever does it.
I think at this point I have to accept the fact that I no longer hang around that kind of immature crowd, and if I wanna test it, I'm going to have to run an experiment in a controlled environment. (read: buy two beers and do it myself.)
TurboThrobber t1_j6ar1tk wrote
There is a bit of plastic inside the bottle cap, when the cap is crimped onto the bottle it creates a gas tight seal.
C_Skadi t1_j6ashc6 wrote
The crimp of the bottle cap engages with the lip of the bottle. This creates many points in contact with the bottle that can exert an equal and opposite force of the gas trying to escape. The cap is held in tension against the force applied by the pressure equally against the cap. The malleable cap will deform if torque is applied, freeing the contents to fill our glasses.
DomesticApe23 t1_j6blc03 wrote
Worth noting that non-twist beer bottle tops are called Crown seals and are the most effective seal for a bottle.
ellipsis31 t1_j6ar0z5 wrote
It absolutely can and does create a seal strong enough to keep the gas contained. Note the inside of the bottle cap, it has a rubber gasket which is pressed against the lip of the bottle mouth to create a seal. Furthermore, this seal doesn't have to resist as much pressure as you might think, most of the carbon dioxide is dissolved in the liquid, the cap only has to contain the pressure in the small head space enough to shift the equilibrium toward dissolved CO2 over gaseous. Same as with soda bottles.