What is the major difference between baking powder and baking soda?

Baking powder and baking soda serve different purposes in baking due to their distinct chemical properties and activation processes. Baking soda, which is sodium bicarbonate, is a pure base that requires an acid (like vinegar, lemon juice, or yogurt) and moisture to begin its leavening action. When baking soda reacts with an acid, it produces carbon dioxide gas, which helps baked goods rise.

Baking powder, on the other hand, contains a combination of an acid (usually cream of tartar) and a base (sodium bicarbonate), along with a moisture-retaining material like starch. It is designed to activate and produce carbon dioxide either with moisture alone if it is "single-acting" or when exposed to heat if it is a "double-acting" type. This means that baking powder can generate leavening gas through two stages: once when added to wet ingredients and again when heated.

Therefore, the statement that baking powder needs moisture and heat to activate, while baking soda needs an acid is the major distinction between the two leavening agents, reflecting their respective roles in baking recipes effectively.