How Baking Soda Works to Make Baked Goods Rise

Baking soda (not to be confused with baking powder), sodium bicarbonate (NaHCO₃), is a leavening agent that's added in food preparation to make baked goods rise. Recipes that use baking soda as a leavening agent also contain an acidic ingredient, such as lemon juice, milk, honey or brown sugar.

When you mix the baking soda, acidic ingredient, and a liquid together, you'll get bubbles of carbon dioxide gas. Specifically, the baking soda (a base) reacts with the acid to give you carbon dioxide gas, water, and salt. This works in the same way as a classic baking soda and vinegar volcano, however, instead of getting an eruption, the carbon dioxide fizzes to puff up your baked goods.

The gas bubbles expand in the heat of the oven and rise to the top of dough or batter into which it's mixed, giving you a fluffy quickbread or light cookies. But you have to be careful! The reaction occurs as soon as the batter or dough is mixed, so if you wait too long to bake a product containing baking soda, the carbon dioxide will dissipate causing your recipe to fall flat.

Waiting too long after mixing to bake can ruin your recipe, but so can using baking soda that's too old. Baking soda has a shelf life of about 18 months. If you're not sure how long the box has been sitting on the shelf, you can test baking soda before adding it to a recipe to make sure it's still good.