I have noticed that some recipes use baking soda, some use baking powder, and some use a bit of each. I don’t know why I never looked into this before, but I got curious to find out why some recipes would call for both leaveners.
Let’s start by defining them. They are both chemical leaveners. Baking soda is simply sodium bicarbonate. It will only work as a leavener if it is combined with moisture and an acidic ingredient (eg. lemon juice, chocolate, fruit, buttermilk, cream of tartar); this combination causes a chemical reaction that produces bubbles of carbon dioxide. As the batter is baked, the heat causes the air bubbles to expand, increasing the product’s volume and making it rise. It starts to react immediately, so any batter made with baking soda needs to be baked as soon as it is mixed to maximize its leavening power.
Baking powder is baking soda that has been premixed with a dry acidic ingredient (monocalcium phospate on my container), so just needs the moisture added to make it work. There are two types of baking powder. Single-acting baking powder has the same reaction as baking soda, where it needs to be baked immediately to maximize the leavening. Double-acting baking powder starts to react immediately, but needs heat in order to maximize its leavening power. Double-acting is what is most readily available in grocery stores now, and allows you to let a batter sit for a while (15-20 minutes) before it needs to go in the oven. Very convenient if you happen to forget to preheat your oven!
And now onto the question of why a recipe would call for both leaveners. I looked this up at the Joy of Baking, and here’s what they say: “When a recipe contains baking powder and baking soda, the baking powder does most of the leavening. The baking soda is added to neutralize the acids in the recipe plus to add tenderness and some leavening.” (Read more: http://www.joyofbaking.com/bakingsoda.html#ixzz2SLm7L0Zv)
So there we have it. If a recipe calls for both, it was probably tested and found to need something to ‘neutralize the acids’ and/or needed a bit of a boost in the rising department. Too much of either leavener on its own can also create an unpleasant taste, but in a balanced combination can make a recipe work beautifully.