And can I use one in place of the other in certain recipes?
Normal double-acting baking powder makes CO2 (thus giving a rising effect) in two ways: when it gets wet, and when it is heated.
Baking soda only makes CO2 when it gets wet.
The acid in a baking powder can be either fast-acting or slow-acting. A fast-acting acid reacts in a wet mixture with baking soda at room temperature, and a slow-acting acid will not react until heated in an oven. Baking powders that contain both fast- and slow-acting acids are double acting; those that contain only one acid are single acting. By providing a second rise in the oven, double-acting baking powders increase the reliability of baked goods by rendering the time elapsed between mixing and baking less critical, and this is the type most widely available to consumers today.
8Note that it's the reaction to acidic ingredients that causes baking soda to produce CO2 which typically occurs during moistening of the dough/batter. Just wanted to clarify so that people don't think that moisture alone will make baking soda react. Jul 20, 2010 at 4:50
This answer is incorrect. Baking soda does not produce CO2 when it gets wet. (Try it!)– SneftelDec 26, 2020 at 8:30
Baking soda is pure sodium bicarbonate, while baking powder includes an acidifying agent (cream of tartar) and a drying agent (starch).
You can substitute baking soda for baking powder if you already have an acidifying agent in a recipe (like buttermilk).
1This answer is right. The 'accepted' answer seems more detailed, but those details are wrong. Thank you for mentioning the need for acid! Aug 2, 2017 at 22:12
You can make your own baking powder using baking soda, cornstarch, and cream of tartar.
1/4 tsp baking soda 1/2 tsp cream of tartar 1/4 tsp cornstarch
That will give you one tsp baking powder. Increase as necessary.
Also, if you don't have all those ingredients, you can use 3 measures of baking powder for every measure of baking soda, although you won't get the same flavor profile with the reduction of acidity from baking soda.
In addition to forefinger's answer, I also believe baking powder has cream of tartar in it, making it more pH neutral.
1Modern commercial baking powders don't typically include cream of tartar but rather different dry acids ( often monocalcium phosphate/sodium aluminum sulfate) that react with moisture and heat which is what makes them "double-acting". Pretty much all commercial baking powders are now double-acting. Jul 20, 2010 at 4:54