[AntoineLavoisier]

I have a 'Soda Stream'. I fill a purpose built, robust plastic bottle to the fill-line and refrigerate. When I want a carbonated drink, I take the bottle to the Soda Stream and dissolve about 200 ml of CO₂ into the bottle. I wait about 10 seconds before I press the button that depressurises the system and allows me to unscrew the bottle from the apparatus. I pour myself a drink. I refill the bottle to the line with tap water and while I do this, I observer water vapour condense and flow out of the neck of the bottle as it is displaced by the tap water.

Why does this happen?

Do I need to post a photograph or short video?

Thank you.

[Borek]

What you observe is a condensation, what condenses is much less clear.

When you fill the bottle you displace air inside - this air is cold. I wonder if the vapor you see doesn't condense from the air _outside_ of the bottle when it gets in contact with cold air _from_ the bottle.