Sarah Shaw, who forgot to forward her school, asks :-
Why did my fizzy drink foam up when I added ice cream to it?
Arthur Williamson, a retired chemical engineer, responded.
Your fizzy drink consists mainly of water with a gas, carbon dioxide, dissolved into it under pressure. When the cap is removed there is excess carbon dioxide in the drink so bubbles want to form. However bubbles only initiate at small, sharp points such as a speck of dust, a hair or a sharp point on the side of the bottle or glass. Next time you open a bottle of fizzy drink notice that the bubbles come in streams, from a very few points in the bottle or glass, points where there is a defect sharp enough to initiate bubble formation.
Your ice cream contains lots of little ice crystals, on all of which bubbles can start to form, so the excess carbon dioxide comes out in a rush of bubbles when ice cream is added to fizzy drink.
You can observe this effect by adding almost any finely powdered material to a fizzy drink. Sugar, salt, baking soda and even crushed ice will all provide nucleation centres and so when added to fizzy drink will cause very rapid bubblying. Perhaps try dropping in one small grain of sugar.
For the same reason the air is clear after rain. The dust particles etc in the air act as nucleation centers for water drops to form and fall with the drop, leaving only clean air behind.