These two have always seemed so similar and it used to confuse me on why every recipe called for specific versions of each, but after doing some research I understand why! Although baking soda is in baking powder, baking powder dilutes it. (This causes more to be necessary when substituting it for baking soda) The properties of baking soda make it good for recipes with acidic ingredients as it prevents things such as heartburn. Baking powder on the other hand adds a fluffiness to food Baking soda’s chemical name is sodium bicarbonate which is also included in Baking powder as said before. If stored in a warm temperature atmosphere, both will eventually lose their potency.
http://culinaryarts.about.com/od/bakingdesserts/a/Baking-Soda-And-Baking-Powder.htm
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