Baking Soda is pure sodium bicarbonate.
Baking powder has cream of tartar and cornstarch added.
You can substitute baking powder in place of baking soda (you'll need more baking powder and it may affect the taste), but you can't use baking soda when a recipe calls for baking powder.
Baking soda by itself lacks the acidity to make a cake rise. However, you can make your own baking powder if you have baking soda and cream of tartar. Simply mix two parts cream of tartar with one part baking soda.
Baking Soda is used in cookies whereas Baking Powder is used for cakes and biscuits.
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