Why does cola foam when you pour it on ice?

This is because the ice is sharp and is something called nucleation. Coke has lots of dissolved carbon dioxide in it, which is what makes it fizzy. When you put it in a glass, you'll notice that the bubbles all come from little spots in one place on the surface of the glass and they stream up in a stream. The reason that they do that is because there'll be a little area on the glass or a tiny imperfection that makes the glass slightly rough and it's easier for a bubble to form there. The carbon dioxide dissolved in the drink starts to come out of solution and come out as bubbles of gas at that point. If you put ice in the glass, which is very very rough and has lots of sharp edges, it creates an even bigger surface area with lots of nucleation sites and you get lots of gas coming out. This is why it becomes frothy.