At least three times now I have seen responses to the question, "What is the difference between baking soda and baking powder?" answered with something like, "Baking powder makes baked goods rise up rather than spread, while baking soda does the opposite."1
To test this, I performed a simple experiment. I made two mixtures. Both contained 150 grams of all purpose flour. To one of these, I added 1 teaspoon of baking powder. To the other, I added one teaspoon of a mixture of baking soda, cream of tartar and cornstarch measured to contain the same amount of bicarbonate of soda as was in the baking powder2.
Each was then blended with 1 cup (228 grams) of water. On an electric griddle, pancakes were cooked from these mixtures, measuring 3 tablespoons of batter per pancake and alternating them on the griddle to compensate for unevenness in the surface temperature. Because they wouldn't be eaten, the pancakes were allowed to cook through from the bottom without flipping them. Flipping might have caused random compression of the height of the pancakes.
After they were cooked, the pancakes were measured for average diameter and maximum height. The results appeared as follows:
|Average Diameter||81.3 mm||83.7 mm|
|Average Height||15.3 mm||14.3 mm|
So, contrary to the sources mentioned, the baking soda pancakes rose slightly higher while the baking powder pancakes spread slightly more. I expect that a larger test than the simple one I did in my kitchen would show that the small differences which I found are not signigicant.
The lessons, as always, are to be skeptical of a lot of the "kitchen wisdom" you read and to try things out for yourself.
1. The sources are Kitchen Wisdom: Harrowsmith's Sourcebook for Cooks (1991); the Iowa Gazette, August 16, 2005; and a cooking website whose URL I can't recall.
2. To get the proper mixture for the baking soda mixture, I referred to the USDA National Nutrient Database for Standard Reference, to determine the sodium level in "baking powder, double-acting, straight phosphate" and in "baking soda". The straight phosphate formulation was used as it most closely resembled the manufacturer's ingredients for the baking powder I had on hand. Based on the sodium ratios, I mixed 3 1/3 teaspoons of baking soda, 6 2/3 teaspoons of cream of tartar and 2 teaspoons of cornstarch. I then used 1 teaspoon of that mixture. The ratio of baking soda to cream of tartar is recommended by Shirley O'Corriher in Cookwise: The Secrets of Cooking Revealed