Question

what ingredient chemically reacts twice? (think pancakes)
flour
baking powder
yeast
baking soda

Explanation:

Baking powder contains both an acid (like cream of tartar) and a base (like sodium bicarbonate). When it gets wet, the acid and base react to produce carbon dioxide gas. Then, when it is heated (as in baking pancakes), another reaction occurs to release more carbon dioxide. Flour is mainly a structural ingredient. Yeast ferments (a single - stage biological reaction in the context of rising). Baking soda (sodium bicarbonate) needs an acid to react (a single - stage reaction in a batter if there is an acid present).

Answer:

Baking Powder