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Re: [Phys-l] fizz-ics



The liquid is supersaturated with CO2 and the can (or bottle) is sealed so the CO2 can't escape. Plus, under pressure the absolute saturation value is different (probably higher). At the top of the can is a small gas pocket in equilibrium with the gas in the liquid. When you open the can, you allow the pressure to rapidly decrease as the gas pocket at the top escapes. This also decreases the saturation value (much like taking warm humid air and bringing it in contact with cold air) and the gas molecules in the liquid come out of solution, join with neighboring molecules into larger pockets of gas which expand because of the lower pressure, pushing the liquid above the pocket up and out.

For cold Coca-Cola which has been sitting calmly, there isn't much desaturation. Dr. Pepper on the other hand has a much lower saturation value or it has much more supersaturation and even cold it will spew out of a bottle. I suspect the former because Dr. Pepper will go flat much quicker than Coca-Cola. Ginger ale is the all time king for retaining carbonation, so it probably wouldn't fizz as much when you shake it up. That would be a fun experiment!

When you shake a can or bottle, you redistribute the gas that was in the top into the liquid where is forms little bubbles, usually on the side of the can or bottle. Given enough time, these bubbles will break loose and float to the surface. If you roll or tap the can enough without random shaking you can get the bubbles to break free, too. If you open the can while the extra gas is down inside the liquid, they expand rapidly, pushing the liquid out.

The increase in the pressure in a bottle is probably from the increase in temperature of the liquid while you hold it in your hand.

"Anthony Lapinski" <Anthony_Lapinski@pds.org> 1/9/2008 9:27 am >>>
I have heard conflicting stories about what happens when you shake/open a
can of soda.

I have gathered that shaking a can of soda does NOT increase the internal
pressure. The system is still in equilibrium. This can be felt by
squeezing the can. However, the soda will spray out when you open the can.
Is this because the CO2 bubbles are forced out of solution (after shaking)
and thus are more likely to come out of the can when it is opened?

If you do this with a 2L bottle (shake, open, recap, shake again), the
internal pressure appears to increase (felt by squeezing the sides).

Can anyone offer a simple physics/chemistry explanation of what's going on?

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