What is the difference between baking soda and baking powder? Can one be substituted for the other?
Baking soda (bicarbonate of soda) and baking powder are both chemical leavening agents used in baking. They react with liquids to form carbon dioxide bubbles which expand when heated, causing baked goods to rise in the oven. Baking soda is a single compound (sodium bicarbonate or NaHC03) and it needs an acid present to cause the reaction to create carbon dioxide bubbles. So recipes that use bicarbonate of soda usually have an acid added, such as vinegar, buttermilk, lemon juice or molasses/treacle. Baking soda reacts very quickly so you will see bubbles starting to form almost immediately in your cake batter.
Baking powder contains sodium bicarbonate but it also contains an acid ingredient (normally cream of tartar) and usually a neutral starch base (such as cornflour/cornstarch or rice flour) to help keep the powder dry and free-flowing. You do not need to add any extra acid to make carbon dioxide bubbles so it is useful for creamed cakes such as victoria sponge. Baking powder is also added to flour to make self-rising (self-raising) flour. We would not recommended to substitute baking powder for baking soda and vice versa in recipes, but you can make your own baking powder by mixing one part baking soda with 2 parts cream of tartar.