Baking powder and Baking soda are both chemical leaveners used to make batters rise. Quick breads, cookies, cakes, and biscuits all get their lift from these two basic baking ingredients.
Baking soda needs an acidic ingredient to react with to release carbon dioxide, the gas that is released in small bubbles to give the baked goods their lift. Acidic ingredients in baking are yogurt, sour cream, buttermilk, molasses, honey, citrus juice, and even vinegar. (This is why you often find white vinegar as an ingredient in red velvet cake). Use baking soda as specified in your recipe. Too much will produce flat baked goods and leave a metallic taste in your cakes or cookies. Baking soda is also known to create browning in baked goods.
Baking powder serves the same purpose as baking soda yet is already mixed with the acidic ingredient needed to produce carbon dioxide when baking. Most baking powders are “double-acting”, which means they produce carbon dioxide when mixed with a liquid and then again, when exposed to heat. Baking powder is used whether or not an acidic ingredient is in the recipe. Be sure to use fresh baking powder in recipes. Old baking powder will produce flat cakes and cookies.
While there is no substitution for baking soda, in a pinch you can replace a teaspoon of baking powder with ½ teaspoon cream of tartar and ¼ teaspoon baking soda.
Store your baking powder and soda in a cool dark place with little humidity. Check the expiration date before using.