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When pouring a standard glass of soda (usually Diet Pepsi), whether on ice or not, the soda fizzes a bit and quickly dies down. When pouring soda over ice cream (again, usually Diet Pepsi but also Root Beer) the soda is particularly fizzy and "foamy" and takes significantly longer to die down.
Why is this? Is there anything I can do to help the soda to not foam up so much or to die down more quickly? (Not interested in flat soda)
When carbonated root beer comes into contact with the ice cream, carbon dioxide bubbles are released. Likewise, the soda helps to free air bubbles trapped in the ice cream. The fat in the ice cream coats all these bubbles, protecting them and allowing them to expand to create the huge heads of foam you see on root beer floats.
https://wonderopolis.org/wonder/why-is-there-so-much-foam-in-a-root-beer-float
To expand on the first point, I believe they are referring to the ice crystals providing a nucleation site for the dissolved gases.
