Baking Powder vs. Baking Soda vs. Yeast
Both baking powder and soda are leavening agents, which means they produce CO2 bubbles when combined with moisture and an acidic ingredient and cause dough to rise. Baking soda is pure sodium bicarbonate. Baking powder contains baking soda, but also has cream of tartar. Cookies often ask for baking soda, while baking powder is often used for cakes, but always check the recipe because this varies a lot. Yeast is a live organism that feeds on sugar and produces CO2 and alcohol in the absence of oxygen. The reason bread rises is because it is suddenly full of CO2. The baking powder/soda reaction is a chemical one, while the yeast reaction is an organic one. There are many different kinds of yeast: instant yeast (used for most things), bread maker yeast (used in bread makers) and regular yeast (used for everything).