Both of these are used as chemical leavening agents in quick breads; essentially helping quick breads rise. Both are white powders and don’t have much of a smell. Though both help create a risen baked good and look similar, you cannot interchange them in recipes. Here’s why.
Baking Soda (Sodium Bicarbonate)
Have you ever seen those science fair volcanoes in action? Do you know how those work? Quite simply: when baking soda comes in contact with an acidic liquid like vinegar, it produces carbon dioxide gas; those bubbles produce the rise that you are wanting in your quick breads. This chemical reaction starts almost immediately once the two ingredients are mixed together and weakens with time. Because of this, you want to use your quick bread batter or dough as soon as possible when using baking soda to get the best results, otherwise, you will have flat muffins or breads even if you did everything else correct.
HINT: Other acidic liquids that can be used to activate baking soda are buttermilk, yogurt, sour cream, and citrus.
Baking Powder is a combination of sodium bicarbonate, a powdered acid (like cream of tartar) and cornstarch. Because of this Baking powder is less concentrated (and less powerful) than baking soda by 1/4th! When moisture is added to baking powder, the same chemical reaction can occur without something to separate the acid from sodium bicarbonate, which is why cornstarch is often added. Cornstarch keeps the baking powder dry and separate so no chemical reactions occur if just added to liquids. That is why the baking powder you buy from the grocery store is double acting- it needs liquid AND heat to activate; the first reaction begins immediately just like baking soda alone with acid, but the second reaction occurs slowly with heat allowing bakers more flexibility with timing.
Baking soda and baking powder have the same objective, help quick breads rise and become light and fluffy but, should NOT be substituted for each other in recipes. They work together as a team in many recipes.
Here is an infographic I made that you can use if you ever forget which one to use in your baked goods. The more you know, the better baker you become!