Baking Powder is a kind of white powder mixed with baking soda and other acidic materials. It is a kind of fast Baking Powder, which can make cakes, buns, steamed buns and other flour products bulked, soft and delicate.
Baking soda which is called Sodium Bicarbonate, bulks the dough by releasing carbon dioxide gas through the decomposition after heated, but the carbon dioxide produced is not much, the steamed buns will not be too bulked. In addition, it is an alkaline chemical, if it is added too much, the dough will turn yellow and be bitter when steamed.
Dry Yeast : an active bacterium that produces carbon dioxide by self-reproduction to make dough bulked, its natural fermenting process .