Self-rising flour is flour mixed with baking powder and salt, so it does not require additional leavening agents, but it is not suitable as a replacement for a rising agent in other recipes. Self-rising flour offers a convenient pre-mixed ingredient for quick breads and other baked goods that do not use yeast as a leavening agent.
According to HowStuffWorks, baking powder is a dry ingredient that manufacturers make by mixing a dry acid, a dry base and a filler. Typical ingredients include sodium bicarbonate, cream of tartar and cornstarch. When mixed with water, the acid and base combine in single-acting baking powder, releasing a gas known as carbon dioxide. These bubbles in the dough cause it to rise, creating a light-textured baked product.
Double-acting baking powder also releases carbon dioxide when mixed with water. A second chemical causes these products to release more carbon dioxide in the heat of the oven. Such products produce even lighter-textured finished products.
Some baked goods call for baking soda, which is sodium bicarbonate. In these recipes, another ingredient in the mix, such as buttermilk or yogurt, provides the acid to create carbon dioxide.
Yeast causes baked goods to rise by fermenting sugar. The alcohol that forms bakes out of the product and the carbon dioxide bubbles cause the bread to rise. Eliminating the yeast from the product eliminates the need for several hours of rising time normally required.