Q: Why do cooked dumplings float while uncooked ones remain close to the bottom of the pot?
A: The reason that dumplings float once they are cooked relates to the presence of baking powder. Usually dumplings are made with plain flour and a large quantity of baking powder. When the mixture is placed into boiling water, the baking powder decomposes to liberate carbon dioxide. This becomes trapped within the dough matrix—held together with the gluten in the flour—and therefore provides buoyancy for the dumplings. You could try making them without any raising agent to see if they remained sunken but I would not advise eating them.
However, this cannot be the only explanation because if you cook meatballs, these too only float once they have been properly cooked and yet they contain no raising agent whatsoever.
David Hauton