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Q & A: soda equilibria

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Most recent answer: 09/27/2010
Q:
I'd like to ask a question about the carbon dioxide in a soda can. It is my understanding that the carbon dioxide is dissolved in the water under pressure, and when we open the can the "fitzz" noise is the carbon dioxide coming out of the water because it is no longer under pressure. In other words, the gas/water solution is equilibrating to room pressure. My question is: If the gas/water solution is in equilibrium while under pressure, then why does shaking the can before opening it cause it to bubble and spew everywhere? There's gas dissolved in water at room temperature, and it doesn't bubble out like crazy when shaking it. How can shaking it so strongly affect a mixture in equilibrium?
- Jake Smetts (age 20)
Champaign, IL, USA
A:
You're right that shaking does very little to change the equilibria. However it does a lot to change the pathway from the high-pressure equilibrium to the low-pressure one.

Let's say the can has been sitting quietly before it's opened. The CO2 is partly on top and partly in solution. When the pressure drops, there are few places for the CO2 in solution to form bubbles. It's hard for a bubble to start in the middle of the liquid, because a very tiny bubble is actually less stable than either a very large bubble or no bubble, due to the surface tension. So the CO2 slowly comes out the top surface.

If you've just shaken the can, it has many bubbles distributed through the liquid. When the pressure drops, they grow rapidly, initially expanding just because the pressure is lower and then because more CO2 flows to the gas from the liquid, just as it does at the top surface. These expanding bubbles make the fizz.

Mike W.

(published on 09/27/2010)

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