A lot of people are good at cooking but not so good at baking. Baking requires real knowledge of proportions and reactions (i.e., chemistry) in order to be successful. One thing all good bakers know is the difference between baking soda and baking powder.
So What's the Deal?
Baking soda and baking powder both create carbon dioxide gas in baked goods. They are leavening agents that work the same way as your third grade science fair volcano. You can thank them for making your cookies light and fluffy and cupcakes rise up over the pan.
If you look at the ingredients, both baking soda and baking powder contain sodium bicarbonate. This is a base that reacts with acids to form carbon dioxide gas, water, and salt.
Baking soda contains only pure sodium bicarbonate. Any recipe that baking soda is used in must include an acidic ingredient, like buttermilk or yogurt. The chemical reaction between the base and acid begins immediately.
When To Use Which
Baking soda is best suited for recipes that include an acidic ingredient, like buttermilk pancakes and biscuits. Baking soda is bitter, so it needs these acidic ingredients to catalyze the reaction and balance the flavors.
Baking powder is more neutral-tasting and requires no additional ingredients to make the reaction start. They're commonly used in cakes and cookies. Single-acting baking powder must be used in recipes intended to be immediately baked. Double-acting can sit out for a while, so it's great to use when making sugar cookie dough that needs to be refrigerated.
Any good baker also knows that sometimes baking substitutions need to be made. If you're out of baking soda, using three times the amount of baking powder will do in a pinch. The taste may or may not be affected.
Since baking soda lacks an acidic ingredient, you'll need to add one if you want to sub it for baking powder. Two parts cream of tartar to one part baking soda can be used as an exact substitution for baking powder. Not ideal, but it'll get you the gas bubbles you need.