Fresh
baking powder
can mean the difference between light fluffy pancakes and flat rubbery pancakes. I learned this first-hand recently when I used a canister that had been unsealed for some time. The date on the bottom of the canister indicated that the baking powder should still be fresh, but upon testing the contents, I learned that it was not.
Baking powder
can lose its effectiveness in hot humid conditions. If you have an unsealed container that has been sitting in the pantry for a while, you may want to test it before adding it to your recipe.
Here is a simple test that will indicate whether you still have fresh, effective baking powder. Put a teaspoon of baking powder in 1/4 cup of hot water. Fresh baking powder will bubble vigorously while older baking powder will barely bubble.
Try it! This simple test may save your recipe.