Baking powder and baking soda are both types of chemical leaveners used in baking. The two have different properties, so they cannot simply be substituted for one another directly.

Baking soda is also known as bicarbonate of soda. It reacts with acidic ingredients, such as buttermilk, yogurt and molasses to create bubbles of carbon dioxide, which causes batter to rise. It starts to work immediately when exposed to the acidic ingredient, so a batter made with baking soda should generally be baked as soon after mixing as possible. Baking soda can also aid in browning during baking.

Baking powder is actually a combination of baking soda, cream of tartar and a bit of cornstarch. Cream of tartar is an acidic ingredient, so it prompts the baking soda to work even when no other acidic ingredients are present in a batter. The cornstarch absorbs moisture and prevents the baking powder from being activated too quickly, or before it makes it into the batter in the first place. It works by releasing CO2 bubbles, just as baking soda does. Many baking powders, and most of those used in the US, are "double acting," meaning that they work once when exposed to moisture and again when exposed to heat. This gives the impression that baking powder doughs and batters will rise higher than those made with baking soda alone, but this is not necessarily the case.

 

Though it seems to rise higher, the baking powder is really only causing a second rise, and will not necessarily produce a higher end product. In addition, baking powder is often reported to cause less "spread" than baking soda during baking or cooking. At the blog Kitchen Savvy, Dave tested this theory by cooking up some pancakes and measuring them for spread and height. His findings showed that the baking soda mixture actually spread less and rose higher than the baking powder one. Of course, one man's pancakes are not hard-and-fast scientific proof, but it does show that it is not always true that baking powder will rise higher/spread less than baking soda. It really depends on the chemical composition of the mixture you're dealing with, not just the leaveners.

There is no substitute for baking soda but, if you must substitute for baking powder, the equivalent to 1 tsp of baking powder can be made by combining 3/4 tsp cream of tartar with 1/4 tsp baking soda. When you're baking, the best bet is to stick with the recipe as it is written, not to try to substitute when it comes to leavening agents.