Whenever I bake something and the recipe calls for baking soda or baking powder, I seem to have one or the other in the kitchen and seldom the right one. Is it really different and could you just substitute them?
Baking soda (like Arm and Hammer) is pure base and will raise the pH of the mixture. It will react to acidic ingredients and will produce CO2 which will cause the mixture to rise. On it's own, it is powerless. It will impart a bitter flavor to baked goods unless neutralized.
Baking powder is baking soda combined with an acid. There are 2 types: single acting and double acting. The single acting reacts as soon as it is hydrated and starts producing gas. You should start baking immediately after mixing this in. Double acting is delayed and will start when hydrated and continue reacting once heated up in the oven. Baking powder is more or less neutral and will not flavor the food so much. Remember: acid is sour and base is bitter.
Can you substitute them? You can use baking powder at any time, it may just end up with a little extra acid and be a bit sour. You can't substitute baking soda when you need baking powder, especially when the mixture is not acidic. So what does this mean? I personally would buy baking powder when shopping because it's more versatile, but in general get what you need and follow the recipe.