What baking powder does
Baking powder is a dry leavening agent that helps cakes and other baked goods to increase in volume. And if your baking powder has gone bad, your tasty treat won’t have the height and airiness you’re looking for.
How to test its freshness
To find out if your baking powder is still good to use, try this trick: Drop a teaspoon of it into a bit of warm water. If the mixture starts to bubble, it means it is still active. If, however, after dropping the baking powder in the warm water your water stays flat, this means the baking powder has unfortunately expired.
So your baking powder has expired and you’re in the middle of baking a cake. Now what? Luckily, there are a few ways to substitute for this ingredient, with the most efficient being a combination of cream of tartar and baking soda. For every teaspoon of baking powder needed in your recipe, substitute it with ½ teaspoon of cream of tartar and ¼ teaspoon of baking soda.