"When `NH_(4)SCN` (colourless) is added to dilute solution of `FeCl_(3)` (light yellow), the colour o the solution becomes deep red but on addition of `NH_(4)Cl` solution (colourless) the red colour faded." Why?

Explanation

Due to the formation of red coloured complex salt `Fe[Fe(SCN)_(6)]` in the reaction between `FeCl_(3) and NH_(4)SCN`, the solution becomes deep red.
`underset(("light yellow"))(2FeCl_(3))+underset(("colourless")(6NH_(4)SCN) hArr underset(("red"))(Fe[Fe(SCN)_(6)])+underset(("colourless"))(6NH_(4)Cl)`
In this case, the forward reaction occurs to a greater extent to attain equilibrium and thus the concentration of `Fe[Fe(SCN)_(6)]` in the solution is high. consequently, the solution assumes red colour. when `NH_(4)Cl` solution (product) is added, the concentration of `NH_(4)Cl` in the solution increases and the equilibrium gets disturbed. at this condition, according to Le Chatelier's principle, the opposite reaction as compared to the forward reaction, takes place to much greater extent to neutralise the effect of increased concentration of `NH_(4)Cl`. as a result, equilibrium shifts to the left, causing a decrease in the concentration of `Fe[Fe(SCN)_(6)]`. this makes the red colour of the solution fade away.