To test your baking soda, use this tip from TheKitchn. Mix together 1/2 cup of hot water (tap is fine), with 1/4 teaspoon of vinegar. Then stir in 1/4 teaspoon of baking soda. If the soda bubbles it’s fine to use, and you can start baking. If not, you’ll need to add it to your grocery list. (You can test baking powder the same way, just leave out the vinegar.) If your baking soda has gone bad, don’t toss it just yet, there are plenty of new uses for it.