Question

What is the brown substance appearing on iron nails after reaction?

Answer 1

Or Copper is less reactive than Iron. So when an iron nail is dipped in a solution of CuSO4, it displaces copper from the Solution, which gets deposited or precipitated as a red or red-brown residue.

Answer 2

after the reaction of corrosion the brown substance starts to form which is called the rust due moisture and oxygen in air