Question

When ferrous ion react with kscn which colour is obtain?

Answer 1

Ferric ions—that is, Fe3+Fe3+ ions—react in aqueous solution with thiocyanate ions—that is, SCN−SCN− ions—to form a dark red colored complex of iron thiocyanate. Note that this reaction is specific for ferric (+3) ions, not for ferrous (+2) ions, or other non-transition metallic elements. (This reaction works for cobaltic ions as well, by the way.)

For the cases A, B and E, the element is sodium, which is not a transition metal: you can scratch those 3 cases. For C and D there’s iron, all right, but in C the iron is in its ferrous oxidation state; only D has iron in its ferric state. So the answer is D.

Now, as to what the dark-colored substance is. The general reaction in aqueous solution is

Fe3++SCN−+5H2O→[Fe(SCN)(H2O)5]2+Fe3++SCN−+5H2O→[Fe(SCN)(H2O)5]2+

which shows the formation of the ferric (penta-aqua) thiocyanate complex, consisting of a thiocyanate ion and 5 water molecules loosely attached to to a ferric ion occupying all the iron’s 6 coordination/conjugation “spots”. The result has a residual positive charge of +2.