CBSE Class 11-science Answered

Addition of aq. HgCl₂ decreases red colour because Hg²⁺ reacts with SCN^– ions to form stable complex ion [Hg(SCN)₄]^2–. Removal of free SCN^– (aq) shifts the equilibrium from right to left to replenish SCN^– ions and thus the intensity of red colour decreases.

Oxalic acid (H₂C₂O₄) also decreases the intensity of red colour. This is because it reacts with Fe³⁺ ions to form the stable complex ion [Fe(C₂O₄)₃]^3–, thus decreasing the  concentration of Fe³⁺ ions.So, to replenish Fe³⁺ ions the equilibrium shifts from right to left and thus the intensity of red colour decreases.