So I have been a chef almost 26 years and I can honestly say that Baking Soda and Baking Powder still confuse me to this day. So, let's explore the world of chemical leaveners.
They both are leaveners which mean that they are added in baking before the cooking process, to produce carbon dioxide which causes it to rise. Baking soda or sodium bicarbonate when combined with moisture and an acidic ingredient like buttermilk,lemon juice and even chocolate causes a chemical reaction that produces carbon dioxide. This expands under the application of heat causing your little muffins to rise. However, this chemical reaction begins immediately upon mixing so it is important to bake soon or otherwise you product will fall.
Baking Powder contains sodium bicarbonate and a little mother's helper called cream of tartar or it's own personal acid, and to make sure the acid does not get outta hand a drying agent(cornstarch) kinda like a babysitter. Now if that is not confusing enough we have single-acting and double-acting baking powder. Single-acting is activated by moisture so you gotta move when mixed to get it baked, double-acting reacts in two phases, once when mixed so you are not under the gun, and the majority of the chemical reaction happens in the oven.
So, I hear you saying "Fine, I get it, but? When should I use them? The primary goal in using these leaveners is to produce a tasty product with a good texture. Baking soda is basic and will result in bitterness unless combined with an acid. Baking powder contains the acid and a base and has a relatively neutral taste, so most the ingredients are neutral in taste, like milk. So go forth and bake your brains out!!!!!
Cooking is like love. It should be entered into with abandon or not at all.~Harriet Van Horne