Baking Powder vs. Baking Soda: the Ultimate Fight

Dear Tricia, What's the difference between baking powder and baking soda? If don't have baking powder, can I use soda instead?

No, lazy bones. Go to the store and buy some baking soda. Even with our current economy, the price of baking soda hasn't gone up so it'll only set you back about 80 cents.
The difference is actually pretty substantial, even though they look so much alike. (Kind of like the kids from "High School Musical." I know they're supposed to be different people, but I just can't tell them apart. Call it being out of touch I guess.)
So lets start with baking soda. In order for any baked goodie to rise in the oven, it needs to produce carbon dioxide. Yeast in breads will produce carbon dioxide bubbles and help make the bread rise. But other leaveners like baking soda and powder need to make that happen too. Sodium bicarbonate is baking soda's real name and it is a naturally occurring salt. When you use it in a recipe, it immediately reacts with anything acidic (like buttermilk, or yogurt, or vinegar) to begin making carbon dioxide. That's why you can't leave a fully mixed batter with baking soda in it just sitting around for a while: it's already working hard for you and you're just letting it go to waste. You have to bake it immediately, slowpoke. Don't save it to bake tomorrow or it won't rise and you'll ruin Christmas.
Baking powder is a much different machine. It is baking soda and an added acid (in the form of a salt crystal, but that's extra info you don't need to know to impress people. Isn't that why you're reading this?) that reacts twice when you're baking. Hence, the "Double Acting" on the label. The first time it reacts with a batter, it starts making carbon dioxide bubbles immediately as it comes in contact with moisture, and then it reacts again when it gets heated up in the oven. Here it is, in a form that's easier to remember:

The Tale of Double Acting Baking Powder

act 1: Baking powder meets batter. They fall in love. Carbon dioxide bubbles form.
act 2: Things get hot in the oven. Carbon dioxide bubbles grow bigger and make the batter rise.

And, scene.

So, if you ran out of baking powder and you're wanting to use soda, go buy the baking powder. You can make a homemade single acting baking powder by mixing baking soda with cream of tartar, but chances are you probably don't have that one in your spice cabinet either. Refer to my "How did Diana Ross' hair get in my eggs?" post and find out why you need to go buy it too.