Key difference: Baking soda is a chemical compound used as a leavening agent in baked goods. Baking powder is generally just baking soda mixed with an acid.

Baking powder is generally just baking soda mixed with an acid, usually cream of tartar and a drying agent, usually starch. Baking soda, also know as sodium bicarbonate or sodium hydrogen carbonate, is a chemical compound, with various applications. In cooking, both baking powder and baking soda are used as leavening agents in baked goods. They release carbon dioxide which causes the batter to rise, ensuring that the cake or bread is soft and light.

Baking soda releases carbon bubbles when it combines and reacts with an acidic ingredient, such as yogurt, chocolate, buttermilk, or honey. This reaction happens immediately, as soon as they are mixed. These bubbles then expand under heat, i.e. in the oven, to make the batter rise.

Baking powder does the same thing, except that it comes with its own acidifying agent. There are two types of baking powders, single-acting and double-acting. Single-acting baking powders start the reaction as soon as they touch moisture, whereas, the double-acting powders are activated by heat.

In baking, baking soda and baking powder are pretty much interchangeable. However, baking soda is basic and will yield a bitter taste unless it is countered by the acidity of another ingredient, such as yogurt, buttermilk, etc. Baking powder has an overall neutral effect on taste, as it has a basic and acidic ingredient. Further differences include:

Image Courtesy: mommiecooks.hubpages.com, culinate.com