Are you doing any baking? If you find yourself having only baking soda and a recipe that calls for baking powder, or vice versa, do you know enough about cooking chemistry to make a substitution? All you need is a bit of cooking chemistry to save the day.
Using Baking Powder When the Recipe Calls for Baking Soda
You can substitute baking powder in place of baking soda, though you will need a bit more baking powder, because it contains additional ingredients. If the recipe calls for baking soda, use 2-4 times more baking powder. So, if the recipe uses 1/2 teaspoon baking soda, use at least 1 teaspoon baking powder. Another suggestion is to replace an acidic liquid in the recipe with a non-acidic one. For example, if you are doing a substitution and the recipe calls for buttermilk, you'll get better results if you change to regular milk.
Using Baking Soda When the Recipe Calls for Baking Powder
You can't directly substitute baking soda if you are out of baking powder. However, you can make your own baking powder, using two parts cream of tartar and one part baking soda. It can get a bit tricky to get the measurements right if you only need 1 teaspoon of baking powder, so what you might want to do is mix up a small batch of homemade baking powder and save the rest for later (stored in a sealed container to keep moisture away). Mix together 1 teaspoon baking soda with 2 teaspoons cream of tartar. Then measure the amount of "baking powder" that you need from that mixture.
Another variation on this recipe is to mix 1/4 teaspoon baking soda, 1/4 teaspoon corn starch, and 1/2 teaspoon cream of tartar. This yields 1 teaspoon of baking powder, which also acts as a double-acting baking powder. You'll get the best results with this version if you use 1 teaspoon of the homemade baking powder for every 1 cup of flour in the recipe.
If you use homemade baking powder, be sure to bake your recipe right away after mixing the ingredients. There are commercial baking powders that will allow you to let a recipe sit for a while before baking, but it's generally a good plan to start heating the food immediately, since the reaction that causes baked goods to rise starts as soon as the wet ingredients are added.
Notes About Baking Substitutions
Substituting leavening agents such as baking powder and baking soda doesn't usually have a huge impact on flavor because these ingredients are present in fairly small amounts. However, you might notice a flavor or texture difference. It won't necessarily be "bad". In fact, you might discover a new favorite recipe!