if you add a teaspoon of baking soda the cookies will be lighter

Baking soda, chemically known as sodium bicarbonate (NaHCO₃), is a white crystalline powder that functions as a chemical leavening agent in baking. Derived from natural mineral deposits or synthesized through industrial processes, this alkaline compound reacts with acidic ingredients to produce carbon dioxide gas, which creates lift and airiness in baked goods. When heated, baking soda decomposes to release CO₂, sodium carbonate, and water, resulting in the characteristic light, porous crumb structure found in cakes, cookies, and quick breads.

Baking soda differs from baking powder in that it requires an acid (such as buttermilk, lemon juice, vinegar, brown sugar, or cocoa) to activate, whereas baking powder contains its own acidic component. The ingredient has a slightly salty, alkaline flavor and can impart a bitter taste if overused or if the acid-to-soda ratio is incorrect.

Baking soda is essential in cookie production, where it promotes spreading and browning while creating tender, cake-like or chewy textures depending on quantity and formulation. It is also used in pancakes, waffles, quick breads, and cakes, particularly in recipes containing acidic ingredients like buttermilk, yogurt, or chocolate. Beyond baking, baking soda serves as a tenderizer for meat, a pH modifier in cooking greens, and a component in batters for fried foods to achieve crispness. In cookie baking specifically, a small amount (typically ¼ to ½ teaspoon per dozen) promotes Maillard browning and produces fluffier results, while excessive amounts create an undesirable soapy flavor.