What is difference between baking soda and baking powder?
The essential difference is that baking soda is pure sodium bicarbonate, and baking powder is sodium bicarbonate combined with acidic components to promote decomposition.
Both may have a few extra components thrown in for baking use. But the primary difference is the additive content. Both of them decompose in the heat of an oven to release carbon dioxide to "aerate" baked goods, but the baking powder can also decompose in moist doughs at room temperature due to the acidic components.
Many forms of baking powder contain sodium bicarbonate combined with calcium acid phosphate, sodium aluminium phosphate or cream of tartar. Baking soda is alkaline; the acid used in baking powder avoids a metallic taste when the chemical change during baking creates sodium carbonate.