The short answer is that baking soda is a single, pure chemical compound, while baking powder is a mixture of that same chemical with one or two others.
(If you must know, baking soda is sodium bicarbonate or bicarbonate of soda, while baking powder is sodium bicarbonate plus an acid or two: tartaric acid or potassium hydrogen tartrate or monocalcium tetrahydrogen phosphate and, if it is “double-acting,” sodium aluminum sulfate.)
A lot of chemicals to put in our food? Well, if you could see the chemical formulas of all the proteins, carbohydrates, fats, vitamins, and minerals in the food, you might stop eating altogether. Chemicals are like cowboys: there are black hats and white hats, and you’ve simply got to choose your friends and enemies intelligently.
So far, even the most contentious natural-food advocates haven’t found anything to complain about in the chemical compounds that reside in your little can of baking powder. In the chemical names above, you may recognize sodium, potassium, calcium, phosphorus, sulfur, and aluminum—all not only harmless, but all (except possibly aluminum) essential to life. And all the carbon atoms, oxygen atoms, and hydrogen atoms in those chemicals pretty much turn into harmless carbon dioxide and water when they hit the oven’s heat.
The key to all of this is the term carbonate in the sodium bicarbonate. A carbonate turns into carbon dioxide gas when provoked either by heat or by an acid. And that’s our reason for using carbonates in baking: They leaven baked goods with carbon dioxide gas. Leaven comes from the Latin levere, meaning to make light or to raise.
Baking soda makes baked goods rise by producing millions of tiny bubbles of carbon dioxide inside the dough or batter, thereby frothing it up. As the froth begins to firm up under the influence of the oven heat, the bubbles become trapped. The result is a light, spongy cake or biscuit instead of a hard lump of dried-out flour paste.
Baking soda, or pure sodium bicarbonate, produces carbon dioxide gas by reacting with any acid ingredients that happen to be present, such as vinegar, yogurt, or buttermilk. It doesn’t give off carbon dioxide all by itself until heated to 518 degrees Fahrenheit (270 degrees Celsius). On the other hand, the reaction with acid begins as soon as the ingredients are mixed; you can see the bubbles forming in buttermilk pancake batter even before you put it on the griddle.
Add a little vinegar to some bicarbonate of soda in a glass and watch it froth up with copious bubbles of carbon dioxide. Vinegar is a solution of acetic acid in water.
Baking powder is baking soda with a dry acid already mixed in, so no other acids are needed in the recipe. As soon as baking powder gets wet, the two chemicals dissolve and react with each other to produce carbon dioxide.
Various chemical compounds are used for the acid in baking powder: monocalcium tetrahydrogen phosphate (usually called calcium phosphate on the labels), tartaric acid, and potassium hydrogen tartrate (cream of tartar) are the most common. To prevent these ingredients from “going off” prematurely in the can, they are diluted with a lot of cornstarch, which keeps them separated from each other until they dissolve in the mixing bowl. Also, they have to be zealously protected from atmospheric moisture by keeping the powder in a tightly closed container.
Add some baking power to a little water and watch it froth up with bubbles of carbon dioxide. If it doesn’t, your baking powder is dead because it became moist and spent its reaction slowly while in the can. Toss it out and buy some fresh.
In most cases, we don’t want our baking powder to release all of its gas bubbles as soon as we mix the dough or batter, before it has hardened up enough to trap the bubbles in place. So we make our baking powder “double acting,” which means that it releases only a portion of its gas upon mixing, and the rest only when the temperature gets high enough in the oven. Double-acting baking powders (and most of them are, these days) usually contain sodium aluminum sulfate, which might be thought of as a high temperature acid.
Baking is a very complicated business; a lot more chemistry is going on than just the leavening. Over the years, different leavening agents have been found to work best in different recipes, from pancakes to biscuits to a million kinds of breads and cakes. The exact times and temperatures at which the bubbles are most advantageously released have been carefully worked out by trial and error. So no substitutions in the recipes, please.
Ironically, nobody likes a pancake that’s flat as a pancake.