BAKING POWDER, a prepared mixture to replace yeast in baking, containing as active ingredients, customarily, sodium bicarbonate (baking soda), and either potassium acid tartrate (cream of tartar), tartaric acid, or potassium aluminium sulphate (alum) or other material which, when moistened, acts as an acid to liberate carbon dioxide from the baking soda. Baking powders frequently contain starch, flour or other inert powdery materials, which serve to prevent premature reaction between the other ingredients.