Originally Posted by Andy M.
Baking powder is a combination of baking soda, cream of tartar, salt.
Actually, most baking powers are a combination of baking soda, cream of tartar and corn starch. Double acting baking powder contains another acidic ingredient (calcium or aluminum acid phosphate or citrate) which reacts to heating. The corn starch is a moisture absorber absorbs moisture to retard the chemical reaction of the baking soda and cream of tartar. Technically cream of tartar, calcium and aluminum phosphate and citrate are chemically salts, but there is no table salt (sodium chloride) in baking powder. Table salt would attract moisture and make the powder clumpy and would prematurely start the chemical reaction.
Here is a good site about baking powders: