Baking Soda: is a chemical compound that consists of mainly sodium bicarbonate. When heating this compound carbon dioxide as well as sodium carbonate is released. The sodium carbonate when released into the mixture creates a specific taste, odor and color that is normally not preferred in many recipes. In order for baking soda to function appropriately as a leavening agent, it must also be combined with acidic ingredients.
Addition of acidic ingredients prevents the release of sodium carbonate. Such ingredients would include cream of tartar, fruits, vinegar, lemon juice, fruit juices, buttermilk, honey, etc. The ultimate chemical reaction produces carbon dioxide gas causing the batter/dough to rise. Read more about different types of Leavening Agents here