baking soda vs. baking powder

June 9, 2009

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.

3 Responses to “baking soda vs. baking powder”

  1. Ali Says:

    I’m still confused: If they’re essentially the same, why do some recipes call for baking powder,and some for baking soda? Can bakers everywhere start ignoring these instructions and just use whichever they have in the cabinet?

  2. Brian Says:

    Interesting. I didn’t know the difference, either.

    I’d also like to prompt another post in which you expand your definition of ‘acid’ to include compounds that contain no protons, like some of those you mention here — monocalcium phosphate, sodium aluminum sulfate, sodium aluminum phosphate, etc. Just keeping you in check. Ali told me to.

  3. Drausin Says:

    Actually, Brian, if you look up the definition of a Lewis Acid, you’ll find that it in no way requires a proton. We studied that in my regular sophomore chem class. Have you gotten there in your Chemistry Ph.D coursework yet?

    Also, for the record
    – monocalcium phosphate – Ca(H2PO4)2

    – sodium aluminum phosphate – NaAl3H14(PO4)8·4H2O

    – sodium aluminum sulfate – AlNa(SO4)2·12H2O, which hydrolyzes water to get H+ ions

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