What happens if you use baking powder instead of baking soda?

baking powder is baking soda and an acid. Today most baking powder is double acting, which means it rises once when it gets wet and again when it gets hot. The answer to the question depends on what your substituting in. Sometimes baking soda is used to adjust the pH (acid or base level) of a dish. Baking powder won't really help there.

If you use the same amount of baking powder instead of baking soda in a cake it will not rise properly.

I made two batches of banana bread. One using baking soda, as called for in the recipe. The other only using baking powder (without adding any cream of Tartar). They both had the same rise to them. I was very surprised. However, I think it does depend on what type of result you are trying to achieve. If you have time to experiment, then you should. There's no better teacher than experience. That's what I did.