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Baking soda is used in recipes that have acidic ingredients like sour cream, yogurt, chocolate, etc., which activate the leavening action. Baking soda reacts immediately when combined with liquid, so it's important that you bake batters made with baking soda immediately. Also, because of this immediate reaction, some recipes are made with a combination of baking soda and baking powder, which reacts in two phases (some gas is released at room temperature when the powder is added to the batter, but the majority of gas is released when the temperature of the batter increases during the baking process.)