Baking Soda and Baking Powder Aren't the Same

Photo by Miansari66, Public domain, via Wikimedia Commons

Both release the same carbon dioxide gas, but baking soda is pure sodium bicarbonate and needs an acidic ingredient in the batter — buttermilk, yogurt, brown sugar, lemon juice — to react and start lifting. Baking powder already contains that acid (plus a drying agent), so it only needs moisture and heat to work, which is why it can leaven a batter with no acidic ingredients in it at all. Swap one for the other 1:1 and you'll either get a flat, dense bake (not enough acid to trigger straight soda) or a bitter, soapy-tasting one (too much unreacted soda) — if a recipe calls for one and you only have the other, look up a proper conversion rather than guessing.