Re: Why does beer overflow from a bottle when the bottle is struck sharply?

By striking the top of the beer bottle you create a high frequency vibration. 
The vibration can't be heard (just like you can't hear the high frequency sound 
emitted from a dog whistle.) The vibration causes changes in the pressure of 
the beer, which directly effect the solubility of the dissolved CO2 in the 
beer. The insoluble CO2 rapidly expands and causes the beer to gush. You can 
look up rarefaction in a Physics book for a more detailed explanation of the 
phenomenon. If you have access to an ultrasonic cleaner in your chemistry 
laboratory you can demonstrate the effect by adding some beer to a beaker and 
placing the beaker in the water of the ultrasonic cleaner. When you turn on the 
cleaner the ultrasonic vibrations cause the same type of pressure changes in 
the beer, which also result in a gusher. Another experiment to demostrate the 
effect of pressure on CO2 solubility is to place the beaker in a bell jar and 
put it under a vacuum. The vacuum drastically reduces the pressure of the beer 
and changes the solubility of the CO2, also causing a gusher. I will also pass 
along a little advice to you. By striking the top of someone's beer bottle you 
could cause a small piece of glass to fall into the beer. This could have 
serious consequences if the glass is swallowed. With that said I must also 
admit that I also did some equally stupid things in college. I hope that you 
can also survive the college experience when your brain goes into vapor lock. 
Toss back a cold one for me!!