Baking soda releases leavening gas when it reacts with an acid.

Water allows the reacting ingredients to dissolve and interact. An acid supplies hydrogen ions that react with bicarbonate, producing carbon dioxide and water. The remaining dissolved ions depend on which acidic ingredient was used, so the reaction is more specific than simply mixing two white powders.

Carbon dioxide enters bubbles within the mixture. During baking, expanding gases help increase volume while proteins and starches develop the structure that holds the finished crumb. Gas production alone is not enough: the batter or dough also needs to retain those bubbles until its structure sets.

Acidic ingredients include buttermilk, lemon juice, vinegar, cream of tartar, and molasses. Their acidity and water content differ, so they are not interchangeable in arbitrary amounts. A tested recipe balances the soda with its other ingredients instead of treating extra leavener as a guaranteed route to greater height.

Utah State University Extension lists baking-powder substitutions that pair small amounts of baking soda with acidic ingredients. Its specific note about molasses explains that an acid must be present for the soda reaction to release carbon dioxide and make the product rise. That is the relevant mechanism behind many quick breads.

Baking powder packages bicarbonate together with acid ingredients, usually with starch that helps keep the mixture dry and manageable. Baking soda supplies the bicarbonate alone. Substituting one for the other by equal volume changes both the quantity of base and the balance of acid in the recipe.

Double-acting baking powder is formulated to release gas in stages, including a reaction associated with heating. Utah State describes it as containing acid salts that support rising after mixing and again when heated. That staged behavior differs from relying entirely on the acid already present in a soda-based batter.

Replacing an acidic liquid also changes more than chemistry. Buttermilk contributes water and milk components, while molasses contributes sugars, flavor, and color. A substitution must preserve a workable overall formula, not merely produce visible fizz in a cup. The extension substitution list therefore specifies quantities and ingredient combinations.

Unreacted soda can leave an unpleasant alkaline flavor and alter browning. Conversely, an acidic mixture with insufficient leavening may not develop the intended volume. Recipe balance affects flavor and texture as well as the amount of gas, which is why indiscriminately adding extra soda is unreliable.

The headline describes acid-driven leavening, not the only possible chemistry of sodium bicarbonate. Heating bicarbonate can also release carbon dioxide through thermal decomposition, leaving sodium carbonate. That route does not make the acid balance irrelevant in baking, because the alkaline residue can still affect the finished food.

A bubbling reaction and a successful cake are connected but not equivalent observations. The first demonstrates gas production; the second also requires an appropriate structure and timing. Comparing baking soda with popcorn helps separate the source of expanding gas from the material that contains it during heating.