JEE Main Chemistry Question (2026) — Solution

Step-by-step solution

K_2Cr_2O_7 in acidic medium acts as a strong oxidizing agent. It changes its colour from orange (Cr_2O_7^ 2- ) to green (Cr^ 3+ ) when it reacts with a reducing agent. We need to identify the set in which all species can act as reducing agents (i.e., they can be oxidized to a higher oxidation state). In option (A), Fe^ 2+ oxidizes to Fe^ 3+ , Sn^ 2+ oxidizes to Sn^ 4+ , I^- oxidizes to I_2, and S^ 2- oxidizes to S. All of these are reducing agents. In option (B), Fe^ 3+ is already in its maximum stable oxidation state and cannot be oxidized further. In option (C), Sn^ 4+ cannot be oxidized further. In option (D), Fe^ 3+ , SO_4^ 2- , and Sn^ 4+ cannot be oxidized further. Thus, only the species in option (A) will change the orange colour of K_2Cr_2O_7. Answer: Fe^ 2+ , Sn^ 2+ , I^-, S^ 2-