One rises, the other spreads. Walk through the science of these two ingredients.
Credit: Credit: Ana Pelaez/Getty

If you learn nothing else from this post, remember this:
Baking powder puffs (rises).
Baking soda spreads.

Let’s take the example of cookies. (Yum, cookies!)

For perfect cut-out cookies, you’d use baking powder because it allows the dough to rise but doesn’t make your proud gingerbread man look like he had a close encounter with a car tire.

But for chocolate chip cookies, you’d use baking soda because it allows the dough to spread, and you get thinner, crisp edges with a tender center. (Now I’m craving cookies!)

That, in an easy-to-remember nutshell, is the science behind baking powder and baking soda. Of course, I’m not going to leave it there.

Deb Wise, our resident baking expert and Test Kitchen Recipe Developer and Tester, walked through the science of the two ingredients with me: Both are chemical leaveners–that is, they both break down in the presence of moisture and/or heat and release carbon dioxide bubbles. The gas bubbles are trapped by the starch in the batter or dough and cause the baked good to expand while in the oven. In essence, these leaveners are responsible for making baked goods so light, porous, and fluffy.

Baking soda needs an acid–buttermilk, lemon juice, vinegar, or sour cream may be used–to begin reacting, releasing gas bubbles, and rising. Baking soda is also typically responsible for any chemical flavor you might taste in a baked good–that bitter or metallic taste is a sign you’ve used too much baking soda in your recipe, and you have unreacted baking soda left in the food.

Baking powder needs first a liquid (as when mixed into a batter) and heat (from the oven) to react and begin releasing gases. You may see this described as “double-acting” baking powder.

Shelf Lives
Baking powder needs to be replaced every 6 to 12 months (follow the expiration date on the can). Baking soda, however, can last you several years, if stored correctly–that is, in a cool, dry place. Because she is a kitchen nerd, Deb likes to date her new can or box so she can remember when to replace it.

Test your baking soda to see if it’s still a viable product by pouring 2 teaspoons vinegar in a bowl with 1/4 teaspoon baking soda. If the mixture bubbles immediately, your soda is still good for baking. If it makes a paste but no bubble, toss the soda.

Test your baking powder by combining 1/2 cup hot water with 1 teaspoon baking powder. If it bubbles, your baking powder gets the thumbs up. If it doesn’t, thumbs down. Toss it, and get another container.

Credit: Photo: Oxmoor House

Don’t have baking powder? You can actually make your own. For one teaspoon of baking powder, combine 1/2 teaspoon cream of tartar, 1/4 teaspoon baking soda, and 1/4 teaspoon cornstarch. (The cornstarch absorbs any moisture and prevents a reaction before the DIY baking powder is in the batter, so don’t skip it.)

Be sure to sift or whisk together your baking powder or baking soda with your flour and other dry ingredients (such as salt, cinnamon, etc.) before combining it with wet ingredients in your recipe. Otherwise, you might end up with very large holes throughout your baked good. And a nasty bite tasting like you licked the inside of a rusty can.