JEE Advanced 2021 Paper 2

The reaction of K3[Fe(CN)6] with freshly prepared FeSO4 solution produces a dark blue precipitate called Turnbull's blue. Reaction of K4[Fe(CN)6] with the FeSO4 solution in complete absence of air produces a white precipitate X, which turns blue in air. Mixing the FeSO4 solution with NaNO3, followed by a slow addition of concentrated H2SO4 through the side of the test tube produces a brown ring.