Question

what happens when u combine soda, seltzier carbon dixode and baking soda with cake

Answer 1

Should be exciting that's all I could tell you-especially if you add lemon or some other acid

Answer 2

When you say "with cake" do you mean raw cake batter or cooked pieces of cake? Just those seem like they won't react very well, however, if you were to add acid, like MissMaxx suggested, it would be just like in 2nd grade when you combined vinegar and baking soda - it'd be all bubbly.