You can easily test the effectiveness of your baking powder or soda before you start baking. To test baking powder, just mix a teaspoon into half a cup of hot water. If the baking powder bubbles, then it is still good. If not, replace it.
For baking soda, mix a quarter of a teaspoon into 2 teaspoons of vinegar. Once again, you should immediately see bubbling. If you don’t, the baking soda is no longer effective.
Baking soda is rarely labelled with an expiration date, so this is super helpful to know.
