Question

Why is dark ring formed at junction of two liquids?

Answer 1

If the two liquids are immiscible then a dark ring forms marking their immiscibility. As they are diffent in there composition then a layer form between them.

Answer 2

Conc H2SO4 has a density almost twice that of water. Hence forms a separate layer at the bottom. Unless we mix the two solutions they donot mix with each other. 
NO is formed at the point at which the two solutions meet. This is because Conc H2SO4 can react with the nitrate present in the solution in contact with it only to give NO. This will in turn combine with FeSO4 present in solution side to give the brown colour. Because it is formed at the junction only, a ring is formed otherwise, if you shake the solution, you donot get the ring but the entire solution turns brown

hope it was helpful...

please mark as brainliest...