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I've noticed that when adding lemon juice to carbonated water, large 'soapy' bubbles appear on the surface. Since I'm adding another acid to a solution of carbonic acid, I would maybe expect a faster release of $\ce{CO2}$, but the bubbles I'm seeing do not resemble the usual fizzy drink bubbles.

What's going on?

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Lemon juice is more than just citric acid. Have you tried the same experiment with pure citric acid (should be available in the baking section of a supermarket)? – Richard Terrett Jan 19 '13 at 0:57
Thanks, will try to. – Noam Kremen Jan 19 '13 at 18:45

The chemistry you're describing the the one you expect: CO2 is formed, and released from the liquid. However, your question is more about kinetics than about the final product. We can assume that slow release of gas leads to larger bubbles, because nucleation rate for the bubble formation is lower. Now, in fizzy drink, there is a large quantity of carbon dioxide accommodated by the liquid due to pressurization. In your case, it will depend on the concentration of your lemon juice (natural lemon juice has pH ~ 2), and more importantly on that of the carbonated water: I just tried with a few grams of sodium bicarbonate dissolved in a glass of water in my kitchen, and the kinetics and bubble size really depends on initial carbonate concentration. (I haven't been able to make pictures or video, because I had to handle my kids who were very fast drawn to this impromptu experiment.)

Also, I second Richard's comment: there are many additional chemicals in lemon juice, some of which surely act as surfactants… which changes bubble formation and kinetics.

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