Once opened, your baking soda and baking powder should be used within 3-6 months for optimum freshness and power. After that, they're still safe to consume, but they may not work as well (most complaints about biscuits that don't rise are from bad baking powder!) To test your baking soda, you'll just need to add a small amount to a bowl. Then, add a splash of acidic liquid (vinegar or lemon juice will do) and see what happens. If the mixture immediately starts to bubble and fizz, your baking soda is fine to use for baking. If not, its time to buy a new box! To test your baking powder, add about a half teaspoon to a bowl of hot water. If the mixture produces lots of bubbles, your baking powder is still good! If not, head to the store or try making your own using the measurements mentioned above.