It is a reaction of the Phosphoric Acid contained in the coca cola to the milk. Milk contains the protein Casein. When mixed with an acid the protein binds that causes it to curdle. You will have the same effect when adding milk to anything acidic like lemons, vinegar, grapefruit etc.
Phosphoric Acid molecules attach to the milk giving them more density and separate out while the remaining liquid that makes up the milk and cocoa cola now being lighter floats on top. The solid matter is basically milk that has been curdled by the addition of more acidic soda.
For more cool science experiments visit our Cool Science Projects page.
Original Video Posted By Youtube User: maricv 84
About The Academy: Professor Egghead Science Academy was founded in Los Angeles, in 2007. The Academy offers the coolest science and engineering after school programs, in school science workshops, science summer camps and kids science birthday parties! See our programs HERE