Baking Powder - Ingredients Descriptions and Photos
From All-Creatures.org Vegan - Vegetarian Recipe Book: How Mary and Frank and Friends Eat

(Baking Powder) Baking powder is used in baking non-yeast breads, cakes, and muffins, and causes the dough to rise by producing tiny bubbles of carbon dioxide that get trapped in the dough, just as yeast does.  It is a leavening agent composed of different chemical formulations of an acid salt (various), and an alkaline salt, usually baking soda (sodium bicarbonate).  There are two types of baking powder: single acting, which contain such chemicals as calcium hydrogen tartrate (cream of tarter), calcium phosphate, or calcium citrate, which begins to work at room temperature as soon as the wet ingredients are added to the dry ingredients; and double acting, which also reacts at higher temperatures during the baking process, and usually contain an aluminum salt, such as calcium aluminum phosphate, which may be harmful to health because of its aluminum content.  To keep baking powder fresh and to prevent the moisture in the air from starting the chemical reaction, it is best to store baking powder in a tightly sealed container in the refrigerator or freezer.  If you wish to avoid the aluminum, and use only the single acting baking powder, a less expensive substitute can be used: for every teaspoon of baking powder called for in the the recipe, add 1/2 teaspoon of baking soda to the dry ingredients, and 1-1/2 teaspoons of lemon juice or vinegar to the wet ingredients.  Happy baking!