Baking Soda and Baking Powder
Chemical leaveners use acidic compounds mixed with an alkaline compound to produce a reaction that will result in carbon dioxide gas much faster than yeast.
Yeast produces carbon dioxide slowly, over the course of an hour or more, so the dough around it must be elastic enough to contain the gas for most of that time.
Weaker doughs and runny batters will not hold gas bubbles for an extended period of time, so they must be leavened with a faster acting source.
Baking Soda
Baking soda (sodium bicarbonate) is commonly used as a leavener and it provides the alkaline component when mixed with a dough or batter that has an acidic ingredient to react with it. These ingredients are commonly sourdough cultures, buttermilk, yogurt, sugar, molasses, chocolate, fruit juices, and vinegar.
Baking Powder
Baking powders are complete leavening systems that contain baking soda and an active acid in the form of crystals. The ingredients are mixed with ground dry starch to prevent them from reacting prematurely. The reaction begins when baking powder is mixed with a liquid. The baking soda dissolves immediately and begins to react with the acidic components. Most baking powders are “double acting” which means they react when they are initially mixed with liquid and then again when introduced to heat during the baking process.