Gas in beer cans

[RTFishall]

Why if I shake up a can of beer before opening it does it foam up? After all the temperature has not changed and the pressure in the can is the same.

[potterfan3]

From everyone's friend wikipedia....

"the formation of tiny bubbles from the agitation that causes the foam; upon opening, the size of the bubbles will rapidly increase due to the reduction in pressure, resulting in excessive foaming."

[meredith101]

Hmm. Sounds ok.
It could I suppose be misunderstood, as meaning that you have these fixed bubbles that grow when exposed to reduced external pressure. In fact, the LIQUID is exposed to reduced pressure, and the CO2 gas held in solution meets its bubble point, causing more gas to break out of solution....into CO2 gas.

[RTFishall]

If there liquid fully filled the can, ie no gas space in the can, would that make any difference?

[heathfield]

When you shake the can, you're making high and low pressure waves pass through the liquid. The bubbles will form in the low pressure regions, and would do so even if the can were completely filled, with no airspace.

[RTFishall]

Many thanks for all your replies. now I'll have to open some more cans of beer to experiment ;)