# Growth of bubbles in supersaturated liquid

I have read the following section which I don't fully understand (I have no technical background). It's from this article which is about the formation of bubbles of CO2 in soda drinks.

When a bottle of liquid with pressurized CO2 is opened at ambient pressure, Henry’s law dictates that any gas cavity inside it will immediately adopt the saturation concentration at ambient pressure. In response to the concentration gradient, molecular diffusion induces a net flux of gas toward the cavity that makes it grow as a bubble.

I understand that the bottle is supersaturated i.e. the concentration of CO2 in the liquid is greater than the saturation concentration at ambient pressure. That means that CO2 must "escape" the liquid at ambient pressure until the concentration is the saturation concentration.

However I have the following questions:

1. How do the "gas cavities" form in the first place? Are they just little bubbles that spontaneously form in the pressurized liquid that then begin to grow when the pressure becomes ambient pressure?
2. What are the "gas cavities"/bubbles made of? Are they just CO2 or a mixture of CO2 and liquid? Because if they purely consist of CO2 (which I assume), the CO2 concentration in a bubble would be 100%, meaning they would actually decrease in size because of the diffusion out of the bubbles.
• – John Rennie Nov 15 '18 at 11:25

Yes I think it forms spontaneously from local fluctuations of the state of the liquid. Here the liquid must fluctuate temporarily to a higher energy state of liquid + small gas cavity. A fluctuation to a higher energy state of energy E should have a probability proportional to $$e^{-E/T}$$. Only here, instead of the fluctuation disappearing by reverting back to its original state, this instability that you described happens, where the excess gas flows into the bubble and fills it. This final state is lower in energy than the original state, so it will stay in this final state.