Leavening agents produce the gas that makes baked goods rise, and choosing the right one affects both texture and flavor. Baking powder is a complete chemical leavener containing baking soda, an acid (usually cream of tartar), and a buffer (typically cornstarch). Double-acting baking powder reacts twice: once when mixed with liquid and again when heated. The general guideline is about 1 teaspoon of baking powder per cup of flour. Baking soda, by contrast, is pure sodium bicarbonate—a base that requires an acidic ingredient like buttermilk, yogurt, lemon juice, brown sugar, honey, or chocolate to react and produce carbon dioxide. It acts immediately, so batters should go into the oven quickly after mixing.
The difference between baking soda and baking powder matters: baking soda needs an added acid to work, while baking powder contains both the base and the acid and is self-contained. Baking soda is roughly three to four times stronger than baking powder, and using it without enough acid produces a soapy, metallic taste. Cream of tartar (potassium bitartrate), the acid commonly used in baking powder, is itself a byproduct of winemaking; in baking, it also stabilizes whipped egg whites in meringues and prevents sugar crystallization in candy.
Yeast is a living organism—specifically Saccharomyces cerevisiae—that ferments sugars to produce carbon dioxide gas and alcohol. The CO₂ gets trapped in the gluten network, causing dough to rise, while the byproducts contribute complex flavors. There are two common forms: active dry yeast must be dissolved in warm water (105–115°F) before use and works more slowly, while instant yeast can be mixed directly with dry ingredients, rises faster, and requires about 25% less by weight. Both produce equivalent results when used properly. Yeast is most active between 75 and 95°F and is killed at temperatures above 140°F, so overly hot liquid is the most common reason a yeast dough fails to rise.