So what exactly is the difference between baking powder and baking soda?
Baking powder is quite often just baking soda (a weak alkali) + cream of tartar (an acid salt) + cornstarch (an inert starch).
Baking soda needs an acid elsewhere in the ingredients list to neutralize it. (Otherwise it will leave a weird alkaline flavor behind.) Recipes that call for baking soda usually also call for something acidic like buttermilk or lemon juice to do the neutralizing.
In the case of baking powder, the cream of tartar is the neutralizing agent.
So basically, pay attention to whether the recipe calls for baking powder or baking soda. No fooling around with substituting one for the other, either. A recipe with baking powder likely won't have other acidic ingredients, as acidic ingredients need to be neutralized with baking soda.
For more info, you should check out The Kitchn's post on the matter! It's even more detailed.
You're welcome :)