Baking soda is sodium bicarbonate; pure and simple. Baking powder is a mixture of sodium bicarbonate and acid salts. The exact mixture depends on whether the baking powder is single acting or double acting. I have actually never used single acting baking powder, but it is traditionally composed of sodium bicarbonate, tartaric acid (also known as potassium bicarbonate or Cream of Tartar) or calcium phosphate and either flour or cornstarch to dilute the mixture. A single acting baking powder works at room temperature; micro air bubbles that are introduced into a recipe expand as the baking soda reacts with the tartaric acid, raising the mixture before the baking process.
Double acting baking powder, the most common type now, has both a low temperature acid salt that performs the function of raising the mixture before it goes into the oven; and high temperature acid salt that expands the mixture once it has been heated in the oven. Typically high temperature acids are made from aluminum salts. The use of aluminum in all food products is somewhat controversial given the links between Alzheimer’s disease and high levels of aluminum, but if you search hard enough, you can find double acting baking powder that doesn’t contain aluminum. It really pays to read food labels!
Baking powder can be substituted in a recipe by using two parts tartaric acid to one part baking soda. Tartaric acid is a byproduct of wine making. During the fermentation process, the grapes leave a crystal residue in their casks. This residue is harvested and purified into a colorless, odorless powder for use in baking. Wow; the origin of food is really fascinating.
Baking soda; a.k.a sodium bicarbonate can be harvested from natural sources or created in a lab in mass quantities. It was originally used by the ancient Egyptians in its mineral form as soap. The natural, mineral form of sodium bicarbonate is called nahcolite. It is found naturally in Searles Lake and other locations in California, as well as in Egypt.