Baking powder, like baking soda, is a leavening agent that is commonly used in baking. This means that you add them to baked goods before cooking to produce carbon dioxide and cause them to expand.
Baking powder contains sodium bicarbonate, but it also includes an acidifying agent and a drying agent. There are several different types of baking powder. You can purchase single-acting, double-acting, and/or sodium free.
Single-acting powders are activated by moisture, so you must bake recipes which include this product immediately after mixing.
The most common baking powder used is Double-acting powder which reacts in two phases first with liquids and again with heat. When you bake with double-acting powder, some gas is released at room temperature when the powder is added to the batter, but the majority of the gas is released after the temperature of the batter increases in the oven. Because of the two stages, baking of the batter can be delayed for about 15-20 minutes without it losing its leavening power.
Too much baking powder can cause the batter to rise rapidly and then collapse. This will give cakes a coarse, fragile crumb with a fallen center. Too little baking powder results in a tough cake that has poor volume and a compact crumb.