A. Baking powder that has been kept in a dry, air-tight container should keep for up to 18 months. Some containers now have expiration dates to help you.
You can tell if baking powder is still good by dropping some in a cup of hot water. If it bubbles and fizzes furiously, then it’s good to use. If it’s flat, it’s time to buy a new can.
Baking powder helps baked goods rise by bubbling up inside the dough.
Baking soda and baking powder are both used to help baked goods rise. But they work differently, as About.com explains:
“Baking soda is pure sodium bicarbonate. When baking soda is combined with moisture and an acidic ingredient (e.g., yogurt, chocolate, buttermilk, honey), the resulting chemical reaction produces bubbles of carbon dioxide that expand under oven temperatures, causing baked goods to rise. The reaction begins immediately upon mixing the ingredients, so you need to bake recipes which call for baking soda immediately, or else they will fall flat!
“Baking powder contains sodium bicarbonate, but it includes the acidifying agent already (cream of tartar), and also a drying agent (usually starch). Baking powder is available as single-acting baking powder and as double-acting baking powder. Single-acting powders are activated by moisture, so you must bake recipes which include this product immediately after mixing. Double-acting powders react in two phases and can stand for a while before baking. With double-acting powder, some gas is released at room temperature when the powder is added to dough, but the majority of the gas is released after the temperature of the dough increases in the oven.”