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The white fog or smoke that appears as a champagne bottle is opened is condensed water from the atmosphere. It results from the pressure/volume work associated with the change in pressure between the sealed bottle (approx. 5 atm) and the open bottle. The associated cooling is sufficient to condense or even freeze water from the surroundings.
For thermodynamic details and more on champagne see:
- Sparkling Wine, Champagne & Co — Part 2,
ChemViews Mag. 2010.