I notice my baking powder had 2/04 dated on bottom of the can. I assume
this is the expiration date. I'm in the middle of mixing ingredients
for cake. How could I substitute with baking soda or is it safe to
still use it?
Thanks in advance.
The expiry date on baking powder is there because it loses some of its strength over time. While the bicarbonate of soda (baking soda) component of baking powder is fairly stable, the acidic ingredient may be less so, and may break down over time especially if exposed to heat or humidity. Because of this, the strength of your baking powder decreases over time. Also, slowly over time, the chemicals in baking powder combine. Humidity from the air causes them to react in the same way they do when mixed in a batter. For this reason, most commercial baking powder has a shelf life of around a year.
According to Cook's Illustrated, you can test the strength of your baking powder by mixing a half teaspoon into a cup of tap water. It should fizz vigorously. This will only work with fast-acting or double-acting baking powder, however. Slow-acting baking powder reacts poorly at room temperature (see Baking Soda vs. Baking Powder for the differences).
Since the expiry date on your package is over a decade old, the chances are that the baking powder will be extremely weak. You can mix a substitute for baking powder, if you have baking soda and cream of tarter on hand. If a recipe calls for 1 teaspoon of baking powder, that can be replaced by ¼ teaspoon of baking soda and ½ teaspoon of cream of tartar. Some cooks also add ¼ teaspoon of cornstarch to make up the same volume, although that isn't strictly necessary.