Here baking soda is single compound like NAHCO3. But baking powder is mixture of sodium hydrogen carbonate and a solid , tartaric acid.
There are bigger difference in both
Baking soda with tarteric acid is called baking powder
No difference

Baking soda contains one ingredient, sodium bicarbonate. However, Baking powder contains sodium bicarbonate, monocalcium phosphate and either sodium acid pyrophosphate or sodium aluminum sulfate.

Kun durgesh kahan gaya tumhara salt user se chapa ider de diya ratoa ratoa
Fixed number of water molecules present in a formula unit is call.
Difference is ( Water of crystallisation)