It's a common question for baking newbies — what’s the difference between baking powder and baking soda? And it's an important question because while both baking powder and baking soda are used to make baked goods rise (they’re called leavening agents), you can’t substitute one for another in a recipe. Baking soda is sodium bicarbonate. When combined with moisture and an acidic ingredient (like buttermilk or lemon juice), a chemical reaction produces bubbles of carbon dioxide that expand during baking, causing the batter to rise. This action begins in the bowl, so batters made with baking soda should be mixed and quickly slid into the oven (don’t mix ahead); otherwise, they may rise too quickly and then “fall” in the oven. Baking powder contains baking soda as well as an acidic ingredient (like cream of tartar). Single acting baking powder rises as soon as it’s activated by moisture. It needs to be put in the oven quickly. Double-acting baking powder has two reactions—one when the ingredients are combined in the bowl and another during baking, once it reaches higher temperatures. Because baking powder is already mixed with an acidic ingredient, you wouldn't add ingredients with acid in them. It's important to remember to read the ingredients carefully when baking with natural foods, like cocoa powder. Natural cocoa powder is acidic, so it's paired with baking soda, while Dutch process cocoa powder has been alkalized to remove the acid, and therefore goes with baking powder.