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I poured some Pepsi into a mug, and it foamed, and I poured until the foam was at the top of the glass. I waited a few seconds for the bubbles to go, and the liquid level was revealed to be at around the halfway point. Then I poured Pepsi into the mug again, but this time there were almost no bubbles at the top -- I'd expect proportionally, if first time there was 50% foam, this time there should be 25% foam, which is significantly different from the ~5% bubbles observed. Why is this?

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  • $\begingroup$ Might Chemistry be better suited for this question? $\endgroup$
    – Kyle Kanos
    Feb 21, 2019 at 0:24
  • $\begingroup$ I'm voting to close this question as off-topic because it doesn't appear to be about physics. (At least, I think not; if anyone has a reasoned argument otherwise, I'll reconsider.) $\endgroup$
    – David Z
    Feb 21, 2019 at 1:10
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    $\begingroup$ The question is about the propensity for gas nucleation. @Rick's answer covers two potential physics-related processes. $\endgroup$ Feb 21, 2019 at 2:59
  • $\begingroup$ I think we've answered questions on bubble nucleation before, so I'd say this was on topic. $\endgroup$ Feb 21, 2019 at 8:00

2 Answers 2

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Carbonated beverages, contains carbon dioxide that has dissolved under pressure (usually at twice the atmospheric pressure). When the pressure is released by opening the soda container, the liquid cannot hold as much carbon dioxide, so the excess bubbles out of the solution.

When you first pour the soda, you relieve even more CO2 out the solution by the simple act of pouring it. This means you have less CO2 in the solution that you think you should.

A second reason you have less bubbles is the surface on which it is poured. The soda already in the glass acts as a buffer to the release of more CO2.

This is the reason why soda expels the gas faster when poured over ice than an empty glass.

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    $\begingroup$ Seems like you could test the first hypothesis by doing the second pour in another dry mug: does it foam as much as the first, or is the carbonation tapped out? The second hypothesis, or one closely related to it, seems more likely: there are much fewer potential nucleation sites after the first pour because you've covered the bottom and sides (with its dust and scratches) with liquid. And any grease in the mug that got dissolved would now serve as a surfactant. $\endgroup$ Feb 21, 2019 at 3:01
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the bubbles come from exsolvation of CO2 at seed sites on the insides of the (initially) dry glass. those seed sites are tiny pits and crevices with air trapped in them, which the pepsi will wet out and deactivate upon pouring the first sample. Only the smallest seed sites will remain active (unwetted) by the time of the second pour.

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