Baking soda and baking powder have some similarities, but differences can cause one to make a cake rise, while the other allows the cake to fall flat of expectations. Both baking soda and powder contain sodium bicarbonate. Baking powder, however, contains both sodium bicarbonate and cream of tartar. This means there will be differences in both taste and results.
Sodium bicarbonate tends to produce carbon dioxide (CO2), when mixed with an acidic substance. This accounts for dough or batter bubbling when sodium bicarbonate in baking soda is added to ingredients like sugar, milk, or shortening. However, soda alone tends to become unstable at higher temperatures. So for example, one might see soda in recipes for pancakes, or cookies, because these food items don’t require long baking times.
Recipes that generally don’t take soda, but instead require baking powder generally have longer baking times. This is because the cream of tartar in baking powder acts as a second leavening agent, and takes over when the soda, or sodium bicarbonate, is rendered inactive by long exposure to heat.
It is possible to substitute baking powder for soda, although one may need to add a larger amount of powder to get the same results. However, the reverse is not true. Substituting soda for baking powder will not work in recipes that require high heats or long cooking times. Also, soda has a somewhat bitter taste, which can be hard to mask.
If one is in a bind and the store is closed, baking soda mixed with cream of tartar is essentially “do it yourself” baking powder. Mix two parts of cream of tartar to one part soda to make baking powder. This will also dull the taste of the soda, so it won’t give off a bitter taste.