baking soda vs. baking powder

I was making brownies the other day and, when the recipe called for a teaspoon of baking powder, realized that I didn’t really know how baking soda and baking powder are different. Having taught high school chemistry for the past two years, I found this an egregious gap in my knowledge and so went to Wikipedia’s entries for baking soda and baking powder to get some answers.

Baking soda is a pure chemical compound, sodium bicarbonate (also known as sodium hydrogen carbonate), NaHCO₃. Sodium bicarbonate reacts with acids (compounds that give off hydrogen ions in aqueous solution) to produce gaseous carbon dioxide.

NaHCO₃(aq) + H⁺(aq) → Na⁺(aq) + H₂O(l) + CO₂(g)

Baking soda therefore needs an acid in order to produce the CO₂ necessary to leaven the brownie, cookie, or bread. Ingredients like lemon juice, yogurt, buttermilk, and vinegar do the trick, but if you’re not interested in adding those to your recipe, solid acid salts like cream of tartar (potassium bitartrate, KC₄H₅O₆) also work just fine.

Baking powder contains three main ingredients: baking soda, a starch, and acid salts. The baking soda is for obvious reasons. The starch, which is usually just cornstarch, prevents any buildup of moisture that might start the reaction and make the baking powder less potent.

The acid salts are usually a combination of two different acids, a slow-acting acid and a fast-acting acid (hence the term, “double acting”). The fast-acting acid begins the reaction right away (as soon as the solids are mixed with the liquids), and the slow-acting acid reaction doesn’t really begin until it’s heated in the oven. The double-action gives more margin for error in the time between mixing and baking. Common slow-acting (high-temperature) acid salts are cream of tartar and monocalcium phosphate. Common fast-acting (low-temperature) acid salts are sodium aluminum sulfate, sodium aluminum phosphate, and sodium acid pyrophosphate.