A chemical leavener, baking soda can work miracles in baked goods, causing them to lighten and rise. In most cases, that old box of baking soda sitting in your pantry will work just as well as a new box, but there are exceptions to this rule. Before you make your cupcakes, test the baking soda to make sure it is still effective.
Baking Soda 101
Baking soda, or sodium bicarbonate, is a highly alkaline substance generally made from trona ore. When it comes in contact with an acid, such as vinegar, buttermilk or molasses, it produces carbon dioxide bubbles. During baking, these bubbles cause the cupcakes to rise and become fluffy. Without baking soda -- or baking powder -- cupcakes will be flat and hard.
Baking powder, which contains baking soda, cream of tartar and cornstarch, only has a shelf life of six to 12 months. Baking soda, on the other hand, will last almost indefinitely if it's stored in a cool, dry place. Exposure to heat or moisture can reduce its shelf life. As long as the baking soda was properly stored, you can probably use it in your cupcakes.
Test the Waters
If you can't remember when you bought the baking soda, it's best to test it to make sure it works. Pour a few spoonfuls of vinegar into a glass or bowl. Add a bit of baking soda. If the baking soda bubbles, it still works. If it simply forms a paste, toss it out and buy new baking soda.
If the cupcakes didn't turn out right, despite using active baking soda, the problem might be caused by something else. Baking soda must be activated by an acid, such as buttermilk, fruit juice, vinegar or brown sugar. If the recipe doesn't call for an acid, you should use baking powder instead. The cream of tartar in baking powder eliminates the need for an acid. Make sure you whisk the baking soda into the dry ingredients before combining with the wet ingredients; otherwise, the cupcakes may have large holes in them.
Another common problem is that the cupcakes spread more than they rose. In this case, the problem is your choice of leavener. Baking soda causes baked goods to spread and gives them a chewy texture. Baking powder causes baked goods to rise high and become fluffy. Baking powder usually gives a finer crumb to baked goods than baking soda. This is why most cupcake recipes call for baking powder, not baking soda.