When you tie on that apron and start to cook, you become a scientist. Recipes are created in the kitchen, just as formulas are in the lab. If one ingredient is off or missing, your final result can be affected.
Science is especially important in baking. The combination of ingredients makes your food rise or fall and be soft like a cloud or as hard as a brick. The same rule of thumb applies in carpentry as it does in baking: Measure twice, cut once. Measure! Don't wing it!
Baking soda and baking powder are not one in the same. These leavening agents each have a specific role in the science of your food. Recipes can call for one or the other, or both, but it depends on what other ingredients are included.
Baking soda is sodium bicarbonate and when it's combined with moisture, acid and heat, it causes your baking to rise.
Baking powder contains sodium bicarbonate, but it already has the acid included (cream of tartar) and a base ingredient like starch.
You can substitute more baking powder for baking soda, but not vice versa. Soda needs an acid, like cream of tartar to make your baking rise. The formula is 2 parts cream of tartar with one part baking soda.
If you are unsure of the substitutions, then it's best to just follow the rules of the recipe and get great results!