I get baking powder and baking soda mixed up all the time; I always have to read a quick baking recipe extra close to make sure I don't make that mistake. While substituting one for the other won't make your cookies explode, it can make a huge difference in the quality and taste of your baked goods. But really, what IS the difference between baking powder and baking soda?
Let's start by reviewing the things they have in common: Both of them are leavening agents. They create bubbles of carbon dioxide gas that cause your bread to rise. Yeast does the same thing, but yeast takes hours to do what baking powder and baking soda can do in minutes.
Baking soda is pure sodium bicarbonate. It's four times stronger than baking powder, but it needs an acidic ingredient to get it bubbling. Some examples of acidic ingredients include vinegar, buttermilk, lemon juice, honey, chocolate, and molasses. Because the reaction is immediate, you usually want to add the liquid ingredients last and bake it right away. If you mix the batter too much or leave it sitting for a while, there won't be any bubbling going on in the oven, and you'll end up with flat bread.
Baking powder contains baking soda, cream of tartar, and a starch. The cream of tarter is the acidic ingredient that activates the baking powder once it's wet. The starch keeps everything dry until you add liquids.
There are two kinds of baking powder: Single-acting and double-acting. The single-acting baking powder is like baking soda in that you need to bake it as soon as the batter is mixed.
Double-acting baking powder generates bubbles in two stages. The most intense "bubbling stage" will be triggered when it's up heated in the oven. So, you can safely delay baking the batter for 15 to 20 minutes.
You could use baking powder as a substitute for baking soda, but it's risky. While you'll need more baking powder, too much will not only make your bread taste bitter, it could make your bread rise too quickly and collapse.
You don't want to use baking soda in a recipe that calls for baking powder, at least not by itself. Without an acidic ingredient to activate it, baking soda will do nothing for your bread except make it taste soapy. However, you can make a fine substitute with two parts cream of tartar and one part baking soda. This will work like single-acting baking powder, so you'll want to get it in the oven quickly.
So whenever you make cookies, you're not just baking. You're doing chemistry in your kitchen!