Baking powder, baking soda and cream of tartar all cause baked goods to rise, but there is a definite distinction between the three powdery white substances.
The leaveners are activated when they are mixed with a liquid, which causes them to effervesce slightly and release carbon dioxide gas, which in turn leavens the baked good.
But for this reaction to take place, both acid and alkaline agents must be present. Baking powder contains both acid and alkaline and can be used alone. But baking soda contains only alkaline, which means it must be used in conjunction with an acid of some sort. This can be either in the form of cream of tartar or a liquid ingredient, such as lemon juice, yogurt or buttermilk.
Some recipes contain all three ingredients to achieve a precise balance of leavening, texture and flavor. In a pinch, you may substitute five-eighths teaspoon cream of tartar plus one-quarter teaspoon baking soda for 1 teaspoon of baking powder. [Schettler]
Schettler, Renee. “These May Look Alike….” The Washington Post. 19 May 2004. <www.washingtonpost.com/wp-dyn/articles/A35851-2004May18.html> (24 May 2004).