So, I dug in my brain and did a quick internet search to come up with a little chemistry lesson. Just for the record, I love chemistry, especially as it relates to food science. It would be fun to do a much more in depth study, but I don't have time.
Hopefully, this will be useful and practical to you as I know it will be for me:
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Baking soda is a base that reacts with acid in a recipe to create air bubbles to lift the batter. You know the ol' baking soda and vinegar experiment.
Baking powder is baking soda, an acidic agent(s), and a stabilizing agent(s). The acid-base reaction occurs when the mixture gets wet. Baking powder is useful for recipes that do not have acidic ingredients.
Baking soda and baking powder are used to leaven many recipes including cakes, quick breads, muffins, pancakes, and waffles. Generally, use 1 tsp baking powder per 1 cup flour in these types of recipes. If the recipe uses acidic ingredients such as citrus, vinegar, honey, molasses, buttermilk, or yogurt, use 1/4 to 1/2 tsp baking soda per 1 cup flour.
Practically, a muffin recipe that uses 2 cups of flour can be leavened with 2 tsp baking powder. If the recipe includes citrus, vinegar, honey, molasses, buttermilk, yogurt or other acidic ingredients, use 1/2-1 tsp baking soda.
Happy Cooking!