Baking Soda

Baking soda is pure sodium bicarbonate. When baking soda is combined with moisture and an acidic ingredient—such as yogurt, chocolate, buttermilk, or honey—the resulting chemical reaction produces bubbles of carbon dioxide that expand under oven temperatures, causing baked goods to expand or rise. The reaction begins immediately upon mixing the ingredients, so you need to bake recipes that call for baking soda immediately, or else they will fall flat.

 Baking Powder

Baking powder contains sodium bicarbonate plus it already includes the acidifying agent (cream of tartar) as well as a drying agent, usually starch. Baking powder is available as a single- or double-acting powder. Single-acting powders are activated by moisture, so you must bake recipes that 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 the dough, but the majority of the gas is released after the temperature of the dough increases in the oven.