Baking powder
Baking powder is a dry chemical used in cooking, mainly baking.  Traditional baking powder was composed of a mixture of tartaric acid and bicarbonate of soda (baking soda), a quantity of flour usually being added to reduce the strength .  When dissolved in water the acid and bicarbonate react and emit carbon dioxide gas which expands, producing bubbles to leaven the mixture.

# Modern baking powders
Most modern baking powders are double acting, that is, they contain two acid salts, one which reacts at room temperature, producing a rise as soon as the dough or batter is prepared, and another which reacts at a higher temperature, causing a further rise during baking. Baking powders that contain only the low-temperature acid salts are called single acting. Many recipes call for a process called creaming, where butter and sugar are beaten together to introduce tiny seed bubbles  which the leavening gas will further expand.
Common low-temperature acid salts include cream of tartar, calcium phosphate, and citrate. High-temperature acid salts are usually aluminium salts, such as calcium aluminium phosphate.  They can be found not only in many baking powders, but also in many non-dairy coffee creamers. Excess aluminium in the diet may be detrimental to human health, and baking powders are available without it for people who are concerned and those sensitive to the taste. However, many commercial products, such as muffin mixes and bakery goods, may contain aluminum-based leavening agents.
While various baking powders were sold in the first half of the 19th century, our modern variants were discovered by Alfred Bird. Eben Norton Horsford, a student of Justus von Liebig, who began his studies on baking powder in 1856, eventually developed a variety he named in honor of Count Rumford. August Oetker, a German pharmacist, made baking powder very popular when he began selling his mixture to housewives. The same recipe he created in 1891 is still sold as Backin in Germany. Oetker started the mass production of baking powder in 1898 and patented his technique in 1903.  It was discovered in Ireland.
In 2006 the development of Rumford Baking Powder was designated an ACS National Historical Chemical Landmark in recognition of its significance for making baking easier, quicker, and more reliable."

# Usage
Baking powder is most often found in quick breads like pancakes, waffles, and muffins. Generally, one teaspoon (5ml) of baking powder is used to raise a mixture of one cup (200-250ml) of flour, one cup of liquid, and one egg.  However, if the mixture is acidic, baking powder's additional acids will remain unconsumed in the chemical reaction and often lend an unpleasant chemical taste to food.  High acidity can be caused by ingredients like buttermilk, lemon, yoghurt, citrus, or honey.  When excessive acidity is present, some of the baking powder is replaced with baking soda.  For example, one cup of flour, one egg, and one cup of buttermilk requires only 12 teaspoon of baking powder -- the remaining leavening is caused by buttermilk acids reacting with 14 teaspoon of baking soda.

# Substituting in recipes
Baking powder is generally just baking soda mixed with an acid, and a number of kitchen acids may be mixed with baking soda to simulate commercial blends of baking powder.  The most common suggestion is to use two parts cream of tartar with one part baking soda.  Vinegar (dilute ethanoic acid), especially white vinegar, is also a common acidifier in baking; for example, many heirloom chocolate cake recipes call for a tablespoon or two of vinegar.  Where a recipe already uses buttermilk or yoghurt, baking soda can be used without cream of tartar (or with less). Alternatively, lemon juice can be substituted for some of the liquid in the recipe, to provide the required acidity to activate the baking soda.
During World War II, Byron H. Smith, a creative inventor in Bangor, Maine, created a substitute product for American housewives, who were unable to obtain baking powder, cream of tartar or baking soda due to war food shortages. Named "Bakewell", a mixture of sodium pyrophosphate and corn starch, the product is still part of regional culinary history. When combined with baking soda, it is essentially the same as any single-acting baking powder, the only difference being that the acid is sodium pyrophosphate.

# Controversy
The use of aluminum compounds as food additives is a source of concern, given the scientific data regarding the safety of aluminum in the diet. High levels of aluminum have been found in the brain tissue of people who suffered from neurological conditions such as Alzheimer's disease. While the use of aluminum pans for frying has become less common due to this concern, aluminum is still being added to the diet via baking powder, aluminum cookie sheets, and other conduits.